Has a former Prime Minister ever broken a three-line whip before?

Question

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.

Answer 1

Edward Heath, (PM in the 1970s) defied an opposition whip in 1997 by abstaining on the vote on the Treaty of Amsterdam. He was the only Conservative to defy the whip on this vote.

(source, page 2 paragraph 4)

Answer 2

It seems that Margaret Thatcher also broke a three-line whip during her time in the House of Lords to vote for an amendment to the European Communities (Amendment) Act in 1993 which implemented the Maastricht Treaty. This amendment would have only allowed Maastricht to be implemented with the consent of the British people in the form of a national referendum.

According to Kevin Theakston in his book After Number 10: Former Prime Ministers in British Politics:

She supported and provided political cover for the Tory Maastricht rebels, stiffening the waverers, in what [John] Major described as ‘a unique occurrence in our party’s history: a former prime minister openly encouraging backbenchers in her own party, many of whom revered her, to overturn the policy of her successor – a policy that had been a manifesto commitment in an election held less than six months before’. Supporters admitted ‘she wanted to defeat the government on Maastricht’. Major would have resigned had the key vote been lost (he narrowly won) and knew his predecessor had tried to destroy him. In 1993 she led the attack on the treaty in the House of Lords, voting against the government on a three-line whip. She became unrestrained in her opposition to a federal Europe and the idea of a single currency.
_{Chapter 9 - Thatcher to Blair}

The particular vote I think this refers to took place in the Lords on July 14th, 1993, on amendment 40. Concluding her remarks in support of a referendum on the treaty, Thatcher made the following comments:

Over the centuries the people and the character of our people have shaped our parliamentary institutions, and not the other way round. People have been the great bulwark against over-mighty rulers and the surest defence of the rights of individuals. Their powers are the heart of our nationhood. The majority of our people want Britain to be in Europe, and so do I. They want to keep intact our Parliament too and they do not want to diminish its powers or its authority or its prestige. In my view, we have surrendered too many powers already. We should surrender no more unless the people wish it. It is the people's turn to speak. It is their powers of which we are the custodians.
_{Baroness Thatcher - Hansard}

She went on to vote 'Content' on the amendment, but was defeated 176-445. The Labour Party had also imposed a three-line whip to vote against the amendment, meaning that it was fairly doomed.