Parliamentary privilege exists to protect members of Parliament from outside interference, so as to enable them to perform their constitutional functions effectively. The most important privileges held by MPs (or members of the House of Lords) are the rights (i) to freedom of speech and (ii) to control their own composition and procedures (‘exclusive cognisance’).
Acts done within the acknowledged scope of privilege are not reviewable by the courts (unlike acts done by royal prerogative). However, where Parliament and the court disagree on the scope of privilege, there is the possibility of a constitutional clash because Parliament claims to be the sole interpreter of its own privileges. In practice, Parliament can resolve any such conflict by passing legislation to define the scope of its privilege.
Within Parliament, issues of breach are dealt with by the Committee of Privileges, a Commons select committee which makes recommendations which are nearly always accepted by Parliament.
i) Freedom of speech
Article 9 of the Bill of Rights 1689 establishes that ‘the freedom of speech, and of debates or proceedings in Parliament ought not to be impeached or questioned in any court or place out of Parliament’. This privilege exists to enable parliamentarians to carry out their functions without fearing civil or criminal proceedings for things they have said.
- Scope: The privilege covers core proceedings such as debates and questions in parliament. But it may also extends to words and actions outside of parliament which are necessary to protect those proceedings, such as when an MP communicates with a constituent about proceedings in parliament. The exact scope can be hard to define but privilege clearly protects an MP from being sued for libel or for contempt of court arising from things they have said in parliament. If such a claim were brought the court should strike it out as disclosing no cause of action.
- Interpretation: Pursuant to the nature of the privilege Parliament asserts its right to define its scope. Erskine May, named after Sir Thomas Erskine May a 19th century Clerk of the House of Commons, entered its 25th edition in 2019. This work, formally called Parliamentary Practice, is considered particularly authoritative on the scope of parliamentary privilege. In practice, since article 9 of the Bill of Rights is a statute, the courts will often be called upon to interpret its scope.
- Interpreting Acts of Parliament: article 9 used to mean that the courts would not have regard to statements in Parliament when construing the meaning of legislation. But in 1993 the House of Lords relaxed this rule in Pepper v Hart so that the courts may now have regard to statements in Parliament made by ministers or other promoters of a bill, but only where an Act of Parliament is ambiguous or obscure.
ii) The right to control its own composition and procedures – exclusive cognisance
An aspect of parliamentary sovereignty is parliament’s right to control and govern its own affairs, a right sometimes summed up in the expression ‘exclusive cognisance’.
- Scope: Parliament decides:
- its procedures,
- whether they have been breached, and
- if so, what the consequences will be.
- Limits on the scope: Parliamentary privilege does not protect MPs from the jurisdiction of the courts to try them on criminal charges. After a scandal in 2009 concerning alleged false claims made by MPs for parliamentary expenses, particularly regarding second homes in London, the Supreme Court ruled that the criminal courts could examine these allegations without adversely affecting the core or essential business of Parliament.
- Parliament has the right to discipline its members, such as by suspending them for misconduct. It even has the right to hold the government in contempt of Parliament as it did in 2018 after Parliament passed a motion demanding that the government disclosed legal advice from its Attorney General, which the government declined to accede to until a contempt motion had been passed.
- Parliament has the right to punish non-members, for contempt of Parliament, such as by issuing a reprimand if, say, a constituent were to attempt to bribe an MP.
- The courts will not question the validity of an Act on the basis that incorrect procedures were followed by Parliament.
 Pepper v Hart  UKHL 3.
 R v Chaytor (David)  UKSC 52.