Jeremy Corbyn will never step aside to allow an alternative candidate to lead a caretaker government because “the rules are the rules”, the shadow chancellor, John McDonnell, has said.

Opposition leaders have held a series of meetings in recent weeks to discuss how to prevent a no-deal Brexit – and have not ruled out a vote of no confidence in the government if Boris Johnson tries to force through a no-deal departure on 31 October.

If Johnson loses, some MPs believe an alternative government could be formed, to extend article 50, and perhaps to trigger an EU referendum before calling a general election.

The Liberal Democrats leader, Jo Swinson, has suggested Corbyn would be unlikely to secure the necessary cross-party support and should step aside in favour of a less contentious candidate.

Several other names have been mooted, including the former foreign secretary Margaret Beckett and the veteran Conservative Ken Clarke, who has had the whip removed for voting against the government.

But Labour rejects that idea, believing that, as leader of the opposition, Corbyn is the clear alternative prime minister. Asked if someone other than Corbyn could lead such a government, McDonnell said: “No, the rules are the rules”.

He suggested Swinson might change her mind, adding: “I’m a great believer in the powers of conversion.”

A senior Labour source said: “People who want a government of national unity are playing fantasy football teams. It’s absolute, complete nonsense. Not only would no such thing ever be agreed, but the idea that such a government could exist for six months with no mandate is pure, utter nonsense.”

Instead, Labour is determined to push for a general election as soon as a no-deal Brexit has been averted.

Several Labour MPs at Monday night’s meeting of the parliamentary Labour party told Corbyn they would prefer a referendum to be held before a general election.

That argument has previously been made by the deputy leader, Tom Watson, and the idea of attaching a “confirmatory” referendum to any Brexit deal the prime minister secures from the EU27 is again being discussed. But the Labour leader has repeatedly rejected that idea.

Opposition MPs and Conservative rebels banded together last month to pass the Benn Act, aimed at forcing Johnson to request a Brexit delay if he fails to pass a deal by 19 October.

But the MPs have widely differing views about how that extension should be used – with some hoping Brexit can be reversed, and others that the time could be used to negotiate a deal.

Instead, Labour has mooted the idea that Corbyn could lead a caretaker government for a matter of days: just long enough to request an extension to article 50 and call a general election. The SNP leader in Westminster, Ian Blackford, has repeatedly made clear that his party could support that idea.

Asked whether Swinson could change her mind and support a Corbyn-led government, a Lib Dem spokesman said: “Jo is a great believer in the power of mathematics. Jeremy Corbyn does not have the numbers and needs to make clear who he would support if we need an emergency government.”

Corbyn stamped his authority on Labour’s Brexit policy at last week’s party conference in Brighton, persuading delegates to back his chosen course of fighting a general election, without picking in advance which side Labour would take in a future referendum.

With the Conservatives poised to campaign on the slogan “Get Brexit done”, Corbyn’s allies are concerned that becoming a remain party would jeopardise votes in leave-supporting constituencies.

But several key shadow ministers, including Keir Starmer and Emily Thornberry, have made clear they want the party to adopt a remain position.

Thornberry was dropped as Corbyn’s regular stand-in for prime minister’s questions (PMQs) after criticising the party’s message in May’s European elections.

With Johnson due to deliver his party conference speech in Manchester at the same time as PMQs on Wednesday, Dominic Raab will appear for the government – and Corbyn will be represented by the shadow home secretary, Diane Abbott.