‘Crossing the Floor’ describes a Member of Parliament changing their political party allegiance.
The term comes from the layout of the House of Commons Chamber, where government and opposition parties sit opposite each other. As a result, an MP who switches party allegiance from the governing to opposition party (or vice versa) literally moves across the room to sit with their new party. This practice dates back to at least the 17th century, when loose party groupings began to emerge.
It can happen either on an individual level or collectively, usually as the result of a major split within a pre-existing party. Some notable instances of ‘crossing the floor’ in the 20th century included Winston Churchill, who crossed the floor from the Conservative party to the Liberal party in 1904, and crossed back in 1924. In 1981, 27 Labour and 1 Conservative MP left their respective parties to form the Social Democratic Party. Some more recent examples include the 8 Labour and 3 Conservative MPs who left their parties to form Change UK in 2019 and the 4 Conservative MPs who have defected to Reform UK since 2024.