The Labour leader, Jeremy Corbyn, said he would not support Mr. Johnson’s call for an election on Dec. 12 because in his view the prime minister was not trustworthy. He also cited more tactical reasons, including the lack of daylight in early December and that university students — who are thought to be more hostile to Brexit — will have gone home for Christmas holidays by then, effectively disenfranchising many students who are registered to vote at the address of their college or university.

“The reason I’m so cautious is that simply, I do not trust the prime minister,” Mr. Corbyn said. “Every promise this prime minister makes, he abandons.”

If Mr. Johnson is on the threshold of an election, that raises the possibility of an end to the Brexit deadlock. Britons could either vote to give Mr. Johnson a majority big enough to push through his Brexit plan, or opt for parties that want a second referendum on withdrawing from the European Union.

Still, even the first step on that road — securing an election — is not assured. The hope of a breakthrough emerged over the weekend when the centrist and pro-European Liberal Democrats and the Scottish National Party said they would be prepared to vote to allow a December election to take place.

The support of these parties could give Mr. Johnson a majority for a bill under which Parliament would disregard the 2011 Fixed Term Parliament Act, which requires a two-thirds majority to the schedule an election.

The Liberal Democrats said they would agree to an election on Dec. 9 — three days earlier than Mr. Johnson had proposed — providing he shelves his plan to try to push his Brexit agreement with Brussels through Parliament.

Before signing off on this, the Liberal Democrats and the S.N.P. would need to be satisfied with the wording of the government proposal, the government’s commitment not to proceed with its Brexit bill and the election date. Mr. Johnson would want assurances that they will not try to amend the bill.