LONDON — A leading opposition lawmaker has thrown cold water on a proposal by the leader of Britain’s Labour Party, Jeremy Corbyn, aimed at preventing Britain from leaving the European Union without a deal.

In a letter sent on Wednesday to other opposition party leaders and rebel members of the governing Conservative Party, Mr. Corbyn urged lawmakers to back a no-confidence vote against the government of Prime Minister Boris Johnson and to allow Mr. Corbyn to lead a “strictly time-limited, temporary government” with the aim of calling a general election and extending the Brexit deadline.

Mr. Corbyn also said he was committed to a public vote on the terms of leaving the European Union, including the option to remain.

But on Thursday, Jo Swinson, the newly elected leader of the anti-Brexit Liberal Democrat Party, rejected Mr. Corbyn’s proposal, saying that while Britain might need an emergency government, he could not be its leader because he would not be able to command a majority in the House of Commons.