On July 13th, 2021, MPs voted against returning the budget for international aid to 0.7% after it was cut to 0.5% in the November Spending Review. Notably, several Conservative MPs voted against the government, including former PM Theresa May, who ended her speech in the Commons with the following statements:
I have been in this House for nearly a quarter of a century. During that time, I have never voted against a three-line Whip from my party. As Prime Minister, I suffered at the hands of rebels. I know what it is like to see party colleagues voting against their Government. We made a promise to the poorest people in the world. The Government have broken that promise. This motion means that promise may be broken for years to come. With deep regret, I will vote against the motion today.
Theresa May - Hansard
This is not the first time that May has voted against the current government - in June 2020 she voted for an amendment to a government motion on the Independent Complaints and Grievance Scheme put forward by Labour MP Chris Bryant and not accepted by the government - but it is, apparently, the first time she's voted against a three-line whip.
It's also not the first time a former party leader has defied a three-line whip - back in March 2020, for example, Iain Duncan Smith was one of 38 Conservative MPs who broke a three-line whip to vote against allowing Huawei to be involved in the UK's 5G network.
A three-line whip, according to the Institute for Government, is:
An explicit instruction to MPs that their attendance is ‘essential’, and that they must vote as instructed. MPs are expected to be in the voting lobbies within six minutes of a vote being called. Express permission is usually required from a party whip to miss a vote, and is rarely granted.
Is this the first time a former Prime Minister has voted against their own party's three-line whip? Let's consider Prime Ministers since 1900, as I'm aware parliamentary records (and the whipping system itself) start to become a little hazy before then.