A contract can be perfectly agreed, properly signed, and supported by consideration — and still leave a court unwilling to enforce it. Illegality is the doctrine that asks a different question from the rest of contract law: not whether the parties consented, but whether giving effect to the bargain would damage the integrity of the legal system. As a solicitor, you'll meet it whenever a client's agreement involves prohibited conduct, an unlawful purpose, or a clause restricting someone's freedom to work — and you'll need to advise on what survives and what falls away.
What this lesson covers:
- The Doctrine and Proportionality — what illegality is, the range of consequences it can produce, and the modern policy-based approach to the defence.
- Statutory Illegality — how to tell whether legislation invalidates a contract or merely penalises the conduct behind it.
- Formation versus Performance — distinguishing an unlawful bargain from a lawful one carried out in an unlawful way.
- Common Law Illegality and Knowledge — the recognised heads of public policy and how each party's knowledge affects enforcement.
- Restraint of Trade and Severance — when clauses limiting trade are enforceable, and how courts sever offending words.
- Raising the Defence — how illegality must be pleaded and when a court can raise it itself.