Define: Likelihood-Of-Success-On-The-Merits Test

Likelihood-Of-Success-On-The-Merits Test
Likelihood-Of-Success-On-The-Merits Test
Quick Summary of Likelihood-Of-Success-On-The-Merits Test

The likelihood-of-success-on-the-merits test is a rule in civil procedure that requires a party to demonstrate a reasonable probability of success in their litigation or appeal in order to obtain a preliminary injunction or prevent the effects of a judgement during an appeal. For instance, if a company is suing another company for patent infringement and wants to halt the sale of the allegedly infringing product during the case, they must show a likelihood of success on the merits. This means proving that they are likely to win the case and that the defendant is indeed infringing on their patent. Similarly, if a defendant has been found guilty of a crime and is appealing the decision to avoid imprisonment, they must demonstrate a likelihood of success on the merits of the appeal. This requires showing a reasonable probability that the appeal will be successful and they will not have to serve their sentence. These examples highlight how the likelihood-of-success-on-the-merits test is utilised in civil procedure to determine whether a party is entitled to a preliminary injunction or to prevent the effects of a judgement during an appeal. It ensures that these measures are not used frivolously and that there is a reasonable chance of success in the underlying case or appeal.

What is the dictionary definition of Likelihood-Of-Success-On-The-Merits Test?
Dictionary Definition of Likelihood-Of-Success-On-The-Merits Test

The likelihood-of-success-on-the-merits test is a civil procedure rule that mandates individuals seeking to prevent a certain action (such as a court decision) to demonstrate a strong probability of winning their case. Essentially, it implies that they believe they can emerge victorious and requests that they not be compelled to undertake any undesired actions until the conclusion of the case.

Full Definition Of Likelihood-Of-Success-On-The-Merits Test

The Likelihood-Of-Success-On-The-Merits Test (often abbreviated as “Likelihood of Success”) is a fundamental concept in the field of injunctions within common law jurisdictions, including the United Kingdom. It serves as a critical criterion for courts when determining whether to grant interim or preliminary relief, such as injunctions, during the pendency of litigation. This test aims to assess whether the party seeking the injunction has a sufficiently strong case to justify the issuance of such a temporary remedy.

Historical Context

The origins of the Likelihood-Of-Success-On-The-Merits Test can be traced back to equitable principles in English law, where courts of equity provided remedies that were not available in common law courts. These equitable remedies included injunctions, specific performance, and other forms of relief that required a more flexible approach to justice. Over time, the principles governing the granting of injunctions evolved, incorporating the Likelihood-Of-Success-On-The-Merits Test as a pivotal factor.

Legal Frameworks

Equitable Remedies and Injunctions

Injunctions are a form of equitable relief that compel a party to do or refrain from doing a specific act. They are categorised into prohibitory injunctions, which prevent a party from taking a particular action, and mandatory injunctions, which require a party to perform a specific act. Given the intrusive nature of injunctions and their potential to cause significant impact on the parties involved, courts employ a rigorous analysis before granting such relief.

Interim vs. Final Injunctions

Interim or preliminary injunctions are granted before the final resolution of the case, aiming to maintain the status quo and prevent irreparable harm. In contrast, final injunctions are issued as part of the final judgment, following a full hearing on the merits. The Likelihood-Of-Success-On-The-Merits Test is primarily relevant in the context of interim injunctions, where the court must make a provisional assessment of the case’s merits.

Application of the Test

The American Cyanamid Principles

The modern approach to the Likelihood-Of-Success-On-The-Merits Test in the United Kingdom is heavily influenced by the landmark case of American Cyanamid Co v Ethicon Ltd [1975] AC 396. This case established a structured framework for courts to follow when considering applications for interim injunctions. The key principles derived from American Cyanamid are as follows:

  • Serious Question to be Tried: The applicant must demonstrate that there is a serious question to be tried, meaning that the case is not frivolous or vexatious. This threshold is relatively low, ensuring that only cases with some merit proceed to the next stage of the analysis.
  • Balance of Convenience: The court must weigh the potential harm to each party if the injunction is granted or refused. This involves considering factors such as the adequacy of damages as a remedy, the impact on third parties, and the preservation of the status quo.
  • Adequacy of Damages: If damages would be an adequate remedy for the applicant, the court is less likely to grant an injunction. Conversely, if damages would not provide sufficient relief, the need for an injunction becomes more compelling.
  • Status Quo: Maintaining the status quo pending the final resolution of the case is a crucial consideration. The court seeks to prevent any party from gaining an unfair advantage through interim measures.
  • Merits of the Case: While the American Cyanamid principles emphasise the balance of convenience, the likelihood of success on the merits still plays a role. Courts are reluctant to grant injunctions if the applicant’s case appears particularly weak or if the respondent’s defence seems overwhelmingly strong.

Case Law Analysis

Recent Developments

While American Cyanamid remains a cornerstone of the Likelihood-Of-Success-On-The-Merits Test, subsequent case law has refined its application. Notable cases in recent years include:

  • National Commercial Bank Jamaica Ltd v Olint Corp Ltd [2009] UKPC 16: This Privy Council decision reaffirmed the American Cyanamid principles but also highlighted that the court should not disregard the likelihood of success entirely. Where the merits are particularly clear, they can influence the decision on interim relief.
  • R (on the application of L) v Commissioner of Police of the Metropolis [2011] EWHC 1186 (Admin): In this case, the High Court emphasised that the likelihood of success should not be ignored, especially where the case appears strong or weak at the interim stage.
  • EL du Pont de Nemours & Co v ST Dupont [2003] EWCA Civ 1368: The Court of Appeal reiterated that the balance of convenience is the primary consideration, but the merits of the case can tilt the balance in close cases.

Practical Considerations

Strategic Use of Injunctions

Parties seeking injunctions must carefully consider the Likelihood-Of-Success-On-The-Merits Test as part of their strategy. Presenting a well-prepared case with strong supporting evidence can enhance the chances of obtaining interim relief. Legal practitioners should focus on demonstrating a serious question to be tried and articulating why damages would be inadequate.

Evidentiary Requirements

Given the provisional nature of interim injunctions, courts rely on affidavits and documentary evidence rather than oral testimony. Therefore, it is crucial to provide comprehensive and persuasive evidence to support the application. This includes highlighting the potential irreparable harm and the inadequacy of damages as a remedy.

Cross-Undertaking in Damages

When granting an interim injunction, courts often require the applicant to provide a cross-undertaking in damages. This means the applicant must compensate the respondent for any losses incurred if it is later determined that the injunction was wrongly granted. The cross-undertaking serves as a safeguard to mitigate the risk of unjust harm to the respondent.

Comparative Perspectives

United States

In the United States, the Likelihood-Of-Success-On-The-Merits Test is similarly integral to the granting of preliminary injunctions. The U.S. Supreme Court case of Winter v. Natural Resources Defence Council, Inc. (2008) 555 U.S. 7 established that plaintiffs seeking preliminary injunctions must demonstrate that they are likely to succeed on the merits, among other factors. This approach aligns with the principles outlined in American Cyanamid but places a somewhat stronger emphasis on the merits.

European Union

Within the European Union, the approach to interim relief varies by jurisdiction but generally incorporates a similar analysis of the merits and balance of convenience. The European Court of Justice has recognised the importance of interim measures in preserving the effectiveness of judicial protection. The likelihood of success on the merits remains a crucial consideration, although it is often balanced with other factors such as urgency and proportionality.


The Likelihood-Of-Success-On-The-Merits Test is a pivotal aspect of the legal framework governing interim injunctions in the United Kingdom. Rooted in equitable principles and refined through case law, this test ensures that courts carefully evaluate the merits of a case before granting temporary relief. The American Cyanamid principles provide a structured approach, emphasising the balance of convenience while still considering the merits of the case. Legal practitioners must navigate this complex landscape with strategic precision, presenting compelling evidence to support their applications and addressing the multifaceted criteria set forth by the courts.

As the legal landscape continues to evolve, the Likelihood-Of-Success-On-The-Merits Test remains a cornerstone of equitable relief, balancing the need for interim measures with the principles of justice and fairness. Whether in the context of commercial disputes, intellectual property conflicts, or public law challenges, this test ensures that interim injunctions are granted judiciously, preserving the integrity of the legal process and the rights of the parties involved.

Likelihood-Of-Success-On-The-Merits Test FAQ'S

The Likelihood-Of-Success-On-The-Merits Test is a legal standard used to determine the probability of a party’s success in a lawsuit based on the merits of their case.

The test is applied by evaluating the strength of the legal arguments, evidence, and applicable laws in a case to determine the likelihood of a party winning on the merits.

Factors such as the clarity and strength of legal claims, the credibility and admissibility of evidence, and the interpretation of relevant laws are considered when applying the test.

While there may be some subjectivity involved in evaluating the strength of legal arguments, the test is generally applied based on objective legal standards and precedents.

The test is not a guarantee of the final outcome, as it is ultimately up to the judge or jury to decide the case. However, it provides an assessment of the probability of success based on the merits.

The test can influence settlement negotiations as it provides parties with an understanding of the strength of their case, which can impact their bargaining position and willingness to settle.

Yes, the test can be applied in various types of legal cases, including civil, criminal, and administrative proceedings, to assess the likelihood of success based on the merits.

Yes, as new evidence is presented or legal arguments evolve, the likelihood of success on the merits can change. It is a dynamic assessment that can be influenced by case developments.

No, the test is just one of many factors considered in a lawsuit. Other factors, such as procedural requirements, jurisdictional issues, and equitable considerations, may also impact the outcome.

The test itself cannot be appealed, but the final decision based on the test can be appealed if there are grounds for challenging the judge’s interpretation or application of the test.

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This site contains general legal information but does not constitute professional legal advice for your particular situation. Persuing this glossary does not create an attorney-client or legal adviser relationship. If you have specific questions, please consult a qualified attorney licensed in your jurisdiction.

This glossary post was last updated: 7th June 2024.

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