LONDON — Amateur lip readers pored over the video clip almost as soon as the British Labour leader, Jeremy Corbyn, shook his head and muttered a few words in disgust.

Had he really called Prime Minister Theresa May a “stupid woman” in the House of Commons? Yes, they agreed, it appeared he had.

The clip quickly bounced around Twitter and onto elected leaders’ phones in Parliament, where members were once again debating Mrs. May’s draft deal for leaving the European Union.

Suddenly, Mrs. May’s Conservative Party, which for months has been torn apart by disagreements over the prime minister’s Brexit plan, was united behind a common cause: forcing Mr. Corbyn to say he was sorry.