What is a three-line whip?

A three-line whip is an instruction by a political party to attend and vote in support of a party’s position in Parliament. A three-line whip means voting with your party is deemed ‘essential’. Permission not to attend may be given by the whip, but a serious reason is needed. Generally, a three-line whip means the ‘slip system’ – where the whips office can ‘pair’ an MP with one from another party who would vote the other way but is also absent – is suspended.

There are no fixed consequences for disobeying the whip, and consequences have ranged from the whip being withdrawn (an MP being effectively expelled from their party) to reduced prospects of future promotion.