British Labour leader Jeremy Corbyn is demanding a block on Brexit during an autumn general election campaign, accusing Prime Minister Boris Johnson of plotting an abuse of power to force no deal.

Mr Corbyn wrote to Whitehall’s top civil servant, Cabinet Secretary Sir Mark Sedwill, amid a growing belief among MPs that Mr Johnson is planning a poll in early November.

Mr Corbyn’s intervention came after Mr Johnson refused to deny well-sourced predictions of an election as early as 1 November, the day after the prime minister insists the UK will leave the EU.

In his letter, Mr Corbyn writes: “You will no doubt be aware of press speculation that, even if parliament votes that it has no confidence in the current Conservative government, the prime minister could attempt to force through a disastrous no-deal Brexit by scheduling a general election after the UK is due to leave the European Union on 31 October.

“Forcing through no deal against a decision of parliament, and denying the choice to the voters in a general election already under way, would be an unprecedented, unconstitutional and anti-democratic abuse of power by a prime minister elected, not by the public, but by a small number of unrepresentative Conservative Party members.”

Mr Corbyn is calling for a ruling on election rules, claiming a convention known as “purdah” suggests that “decisions on matters of policy on which a new government might be expected to want the opportunity to take a different view from the present government should be postponed until after the election, provided that such postponement would not be detrimental to the national interest or wasteful of public money”.

“I would therefore be grateful for your confirmation that, if the UK is due to leave the EU without a deal during a general election campaign, the government should seek a time-limited extension to Article 50, to let the electorate decide and the incoming government to take the next steps on the basis of the voters’ wishes,” Mr Corbyn added.

Mr Corbyn’s letter coincided with an announcement from the Chancellor, Sajid Javid that he plans to fast-track a new “spending round” – cutting it from three years to 12 months – to free up departments to prepare for Brexit.

The aim is to find the cash to fund Mr Johnson’s spending spree since he came to office, including the recruitment of 20,000 extra police officers, additional funding for schools and the new promises on the NHS.

“We will get Brexit done by October 31 and put our country on the road to a brighter future,” said Mr Javid.

“The prime minister and I have asked for a fast-tracked Spending Round for September to set departmental budgets for next year.

“This will clear the ground ahead of Brexit while delivering on people’s priorities.”

Labour’s shadow transport secretary Andy McDonald told Sky News Mr Johnson forcing through no-deal after losing a no-confidence vote would be “anti-democratic”.

“We’ve got to resist it and defeat it,” he said.

Earlier, a firm prediction of a no-deal Brexit was made by the respected academic and pollster Sir John Curtice, interviewed at an Edinburgh Festival Fringe event by the broadcaster Iain Dale.

“It is now too late for us to leave the European Union on October 31 with any kind of deal,” he said. “There isn’t time. Even if the government were wanting to put Theresa May’s deal through, we’ve run out of time.”

On prospects for an autumn election, Sir John said: “The polling evidence is that the Brexit Party vote has basically gone down now to where the UKIP vote was in 2015.

“And what Brexit supporters are saying when they’re asked these hypothetical questions is, ‘if we leave after October 31 and we’ve left with a deal or not, then at least half of them say they’ll switch to the Tories’.

“If on the other hand, the government tries to hold an election before October 31 and we haven’t left, or if it holds an election and it hasn’t left, they say they’ll stick with the Brexit Party.

“So certainly delivering Brexit ought to have a dividend for the Conservative Party. But of course, if we do leave without a deal on October 31, there is a significant debate out there about what the consequences will be.”

He added: “My view is that if the government is forced into an election this side of October 31, it’s going to be a tough call for the Tories.

“If they can wait until afterwards and they deliver Brexit, and the delivery of Brexit is seen to be successful, then so long as the Remain vote remains divided then they should be able to squeeze the Brexit Party votes sufficiently that they can get a majority.”