LONDON — Rising to speak before a half-empty House of Commons on Wednesday, Nicholas Soames, the grandson of Winston Churchill, choked up, fighting back tears.

Mr. Soames, 71, has served in Parliament for 37 years and is a popular figure, known to have a lively sense of humor. But he now was announcing his decision to retire from politics, a day after Prime Minister Boris Johnson had expelled him from the Conservative Party. He had joined a group of 21 Tory rebels who defied Mr. Johnson on Tuesday night by voting for legislation that would pave the way toward blocking a no-deal Brexit.

“I’m truly very sad that it should end in this way,” Mr. Soames said. “It is my most fervent hope that this house will rediscover the spirit of compromise, humility and understanding that will enable us finally to push ahead with the vital work in the interests of the whole country that has inevitably had to be so sadly neglected whilst we have devoted so much time to wrestling with Brexit.”

If Brexit is shaking the foundations of party politics in Britain, the expulsion and resignation of Mr. Soames is a moment that not long ago would have been unimaginable. It is especially searing given that Mr. Johnson, who cast him aside, considers Churchill one of his greatest heroes, and has written a biography of Britain’s wartime leader.