This guide is maintained as a current resource for July 2026 and covers only the laws of England and Wales. Information is for general guidance, not legal advice. Consult a qualified solicitor for advice specific to your situation.
A detailed explanation of common mistake in contract law, including legal tests, key case law such as Bell v Lever Brothers, types of mistake, commercial examples, and when contracts may be rendered void under English law.
Common mistake in contract law occurs where both parties to a contract share the same incorrect assumption about a fundamental fact at the time the agreement is made. This shared error can, in limited circumstances, render the contract void or allow it to be set aside.
In English law, courts are generally reluctant to interfere with contracts on the basis of mistake, reflecting the principle that parties are responsible for the risks they agree to take. However, where a mistake is sufficiently serious and goes to the root of the contract, the law may provide relief.
This area of law is particularly important in commercial contracts involving goods, property, and high-value transactions where assumptions about subject matter or facts can significantly affect obligations and pricing.
Meaning of Common Mistake
A common mistake arises where:
- Both parties are mistaken
- They share the same incorrect belief
- The mistake relates to a fundamental fact
- The fact existed at the time the contract was formed
The key point is that neither party is aware of the error when entering into the agreement.
However, not every shared mistake will invalidate a contract. The mistake must be sufficiently fundamental to undermine the contract's existence or purpose.
Legal Effect of Common Mistake
A successful claim of common mistake may result in the contract being declared:
- Void ab initio (treated as if it never existed), or
- In some cases, voidable or subject to equitable relief
However, modern English law has narrowed the circumstances in which common mistake will render a contract void.
The courts aim to preserve contractual certainty and will only intervene where enforcement would be unjust.
Key Test for Common Mistake
The leading approach to common mistake is based on strict legal criteria developed through case law.
The core question is:
- Does the mistake make the subject matter of the contract essentially different from what the parties believed it to be?
If yes, the contract may be void.
If no, the contract will generally remain valid.
Leading Case Law on Common Mistake
Bell v Lever Brothers Ltd
This case is the leading authority on common mistake. The House of Lords held that a contract is not void for mistake unless the mistake is fundamental enough to destroy the identity of the subject matter of the agreement.
It established that:
- A mistake must relate to something essential
- A mere bad bargain or incorrect assumption is not enough
- The contract remains valid unless the mistake is fundamental
This case significantly restricted the availability of the doctrine.
Types of Common Mistake
1. Mistake as to Existence of Subject Matter
This occurs where both parties believe something exists, but it does not.
Example:
- A contract for the sale of goods that have already been destroyed before the agreement is made.
In such cases, the contract may be void because there is nothing to contract about.
2. Mistake as to Ownership
This arises where both parties assume one party owns something when in fact they do not.
Example:
- Both parties assume a seller owns shares that are in fact no longer valid or transferable.
The legal effect depends on whether the mistake is fundamental.
3. Mistake as to Quality
This is the most common type in commercial disputes.
However, English law is strict here. A mistake as to quality will rarely render a contract void.
Example:
- Both parties believe a business asset is highly profitable, but it turns out to be less valuable.
Generally, this is treated as a bad bargain, not a legal mistake.
Limits of Common Mistake
English courts impose strict limits on common mistake claims.
A contract will not be void if:
- The mistake relates only to quality or value
- One party has assumed risk under the contract
- The contract allocates risk expressly or implicitly
- The mistake does not destroy the contract's subject matter
This approach ensures commercial certainty and predictability.
Effect of Risk Allocation
A key principle is that if a contract allocates risk to one party, common mistake will not apply.
Courts will examine:
- Contract wording
- Commercial context
- Industry practice
- Whether one party guaranteed the existence or condition of the subject matter
If risk is allocated, the contract will usually remain valid even if assumptions prove wrong.
Common Mistake vs Other Types of Mistake
1. Mutual Mistake
Where both parties are mistaken, but not necessarily in the same way. This may overlap with misinterpretation or contract formation issues.
2. Unilateral Mistake
Where only one party is mistaken and the other knows or ought to know.
3. Common Mistake
Where both parties share the same fundamental mistaken assumption.
Each category has different legal tests and consequences.
Practical Examples in Commercial Contracts
Example 1: Sale of Goods
A contract is made for the sale of a shipment that has already been destroyed in transit without either party knowing. The contract may be void due to common mistake.
Example 2: Business Asset Sale
Both buyer and seller believe a business holds a valuable licence, but it has already expired. Courts will assess whether the mistake is fundamental or merely affects value.
Example 3: Property Transaction
Both parties assume a building has planning permission when it does not. This may or may not qualify depending on whether the mistake destroys the contract's purpose.
Remedies and Legal Consequences
Where common mistake is established:
- The contract is treated as void
- Money paid may be recoverable
- Property transferred may need to be returned
- Courts may apply equitable adjustments to achieve fairness
However, restitution may be complex where performance has already occurred.
Risks and Legal Challenges
High Threshold for Proof
Courts require strong evidence that the mistake was fundamental.
Commercial Uncertainty
Courts are reluctant to invalidate contracts, particularly in business contexts.
Overlap With Misrepresentation
Some cases involve overlapping issues of mistake and misrepresentation, which may change the legal remedy.
Financial Exposure
If a claim fails, parties may be left bound by an unfavourable contract.
Common Mistake in Modern Commercial Law
Modern courts continue to apply a restrictive approach, prioritising:
- Certainty in commercial transactions
- Allocation of contractual risk
- Enforcement of written agreements
As a result, successful common mistake claims are relatively rare.
Practical Considerations for Businesses
To reduce risk:
- Clearly define subject matter in contracts
- Include warranties and representations
- Allocate risk explicitly
- Conduct due diligence before entering agreements
- Use termination clauses where uncertainty exists
Proper drafting significantly reduces reliance on common mistake arguments.
Key Takeaways
Common mistake in contract law occurs where both parties share a fundamental incorrect assumption at the time of contracting. English courts apply a strict test, and only mistakes that destroy the identity of the subject matter will typically render a contract void. Most errors relating to value or quality are not sufficient. The doctrine is therefore narrowly applied to preserve certainty in commercial contracts and ensure that only truly fundamental errors justify setting a contract aside.
