The following information was prepared by the Commons Library – to whom thanks.

There are no rules requiring the resignation of an MP who leaves one political party for another. A convention that the Member changing parties does not resign to fight a by-election accords with the arguments of Edmund Burke in the late 18th century. This MP, himself a rebel in a number of policy areas, considered that a Member was a representative rather than a delegate. Historically, the Commons has acted on the principle that all Members of the House of Commons are individually elected, and voters put a “cross against the name of a candidate”.  While decisions on candidates may be affected by their party labels, MPs are free to develop their own arguments once elected, until it is time to face the voters in the next general election.

Are there any sanctions or other forms of discouragement which could be imposed on an MP for leaving the party on whose behalf she/he was elected?

There are no formal sanctions but discouragement comes in many forms, including: reaction of former party colleagues and the electorate. Although Members who change party allegiance during the course of a Parliament, rarely stand for their new party in the same constituency at the following election, crossing the floor does not mean political death.  There are examples of Members being found safe seats by their new party and even taking their place on the front bench.

Is there any pressure to take action against Members who defect?

Occasionally there are calls for Members to resign or for by-elections to be triggered if they change parties. For example, in 2010, it was suggested that “Members should be required to cause a by-election if they defect to a different party from the one on whose manifesto they were elected”.  However, the Government said that such a change would be “a major constitutional reform of the role of Members of Parliament and their independence” and that it had “no plans to do that”.

Who has crossed the floor?

True instances of crossing the floor – leaving the Government for the Opposition or vice versa – are rare. In the last 15 years, just three MPs have moved from the Official Opposition to the governing Party, none have gone in the opposite direction.

• Quentin Davies crossed the floor from the opposition Conservative Party to the governing Labour Party in June 2007.  He was given a peerage in 2010.
• Robert Jackson, left the Conservative Party for the Labour Party, in January 2005.  He left Parliament at the next election, in May 2005.
• Shaun Woodward, who was elected as a Conservative Member in Witney in 1997, crossed the floor in December 1999.  At the general election in 2001, he was elected as a Labour Member in St Helens South.  He went on to serve as Northern Ireland Secretary from 2007-10 in the Labour Cabinet.

Previous examples include Winston Churchill and Reg Prentice who both held ministerial office in two different parties. Churchill left the Conservative Party in May 1904, joining the Liberal Party and becoming a minister in 1905.  He was a defeated Liberal candidate in the 1922 and 1923 general elections, and as an independent in a 1924 by-election.   In the 1924 general election he was elected as a Constitutionalist and became Chancellor of the Exchequer in the Conservative Government following that election.  Reg Prentice was Labour’s Secretary of State for Education in 1974-75 but following deselection by his constituency party, he left for the Conservative Party in 1977.  From 1979-81, he was a Minister of State of in Margaret Thatcher’s first Conservative Government.

The Library has produced a Note – Members’ Changes of Allegiance.  It details moves by Members from one party directly to another and instances of Members losing or resigning a party whip.