Jeremy Corbyn has called on the UK’s most senior civil servant to intervene to stop Boris Johnson forcing a no-deal Brexit in the middle of an election campaign, amid rising signs the country is heading for the polls again this autumn.

The Labour leader wrote to Sir Mark Sedwill, the cabinet secretary, accusing the prime minister of plotting an “unprecedented, unconstitutional and anti-democratic abuse of power”, after it emerged No 10 would be prepared to delay an election until immediately after 31 October if Johnson loses a no confidence vote among MPs.

In his letter, Corbyn demanded urgent clarification of the rules around purdah, which are meant to prevent the government taking major policy decisions during an election campaign.

He asked Sedwill to confirm that if the UK is due to leave the EU without a deal during an election campaign, then the government must seek an extension to article 50 and allow an incoming administration to take a decision about Brexit on the basis of the result.

“Forcing through no deal against a decision of parliament, and denying the choice to the voters in a general election already underway, 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,” he said.

“I am therefore writing to seek your urgent clarification on the proper application of ‘purdah’ rules in such a scenario and the constitutional implications of failing to abide by those rules.”

Corbyn released the letter as No 10 refused to rule out delaying an election until the immediate few days after Brexit on 31 October if one is triggered by MPs voting down Johnson’s government and failing to form another administration.

Asked by the BBC on Thursday if he would resign if he lost a confidence vote, Johnson swerved the question, and stressed the need to leave the EU on 31 October.

“I think that what MPs should do and what I think they’ve already voted to do, when triggering article 50 and reconfirmed several times, is honour the mandate of the people and leave the EU on 31 October,” he said.

He also insisted there was “bags of time” for the EU to “show some flexibility” and agree to ditch the Irish backstop, which he claimed could make the UK into a “satellite state” of Brussels.

No 10 and the Conservative machine appear to be getting into election mode with the appointment of Isaac Levido, a former staffer for Lynton Crosby, as a new director of politics and campaigning.

Quick Guide Who's who - Boris Johnson's controversial backroom team Show Boris Johnson's new backroom team in Downing Street is littered with ex-staff from Vote Leave, supports of controversial lobbying groups like the TaxPayers’ Alliance, and those with links to Lynton Crosby and Mark Textor's C|T Group Dominic Cummings Special advisor to the prime minister Boris Johnson and chief of staff in all but name, Cummings was campaign director of Vote Leave. He had previously campaigned against Britain joining the Euro, and worked for Iain Duncan Smith as director of strategy at the Conservatives, and for Michael Gove as a special advisor in the department of education. Isaac Levido A Lynton Crosby protege, Australian Levido has been hired into Conservative party headquarters as director of politics and campaigning. He has previously worked in Washington for the Republicans, and contributed to the Tory campaigns in 2015 and 2017. Earlier this year he worked on the Liberal party’s surprise election success in Australia, where the party’s Facebook videos were watched at triple the rate of the Labor opposition videos during the election campaign. Lee Cain Head of communications for Johnson and responsible for determining the Conservative government’s message in public. He was the head of broadcast for the Vote Leave campaign and had government jobs, including at No 10, before joining Johnson at the Foreign Office. His most public role, though, was dressing up as a chicken in 2010 to heckle Tory politicians. Rob Oxley Press secretary at Downing Street, Oxley has previously served as an advisor to Home Secretary Priti Patel, and worked alongside Cain as press officer for the Vote Leave campaign. Oliver Lewis Now the Johnson government’s Brexit policy adviser, Lewis was Research Director at Vote Leave. Munira Mirza Heading up Johnson's policy unit, Mirza was his deputy mayor for arts in London for eight years. She has links to a circle of former Revolutionary Communist Party supporters who wrote for Living Marxism, before morphing into libertarian provocateurs involved with Spiked online magazine. She co-founded of the Manifesto Club, a pressure group challenging the “erosion of public freedoms”. Chloe Westley A digital adviser to the administration, Westley worked at both Vote Leave and the TaxPayers’ Alliance. She found fame on Twitter as @LowTaxChloe making videos attempting to mock attempting to mock Corbynite socialism. She was involved in Turning Point, a student pressure group dedicated to “free markets, limited government and personal responsibility” which drew attention when at one of its launch events American conservative Candace Owens appeared to praise HItler’s approach to making Germany great. Westley herself has praised the work of far-right, anti-Islam politician Anne Marie Waters. Ross Kempsell Former Guido Fawkes chief reporter and Talkradio political editor Kempsell has joined Johnson’s team as a special adviser focused on reform of Whitehall and the public sector just weeks after his interview with the prime minister during his leadership campaign prompted Johnson to rattle off an anecdote about making and painting cardboard buses as a hobby. He also was the interviewer when Johnson promised Brexit would be carried out “do or die” by 31 October. Danny Kruger Has moved from being the Department for Digital, Culture, Media and Sport’s expert adviser on charities to the role of political secretary. He stood down as a Tory candidate in 2005 after causing controversy by saying he thought there should be a “period of creative destruction in the public services”. He argues that cannabis should be decriminalised. Blair Gibbs Previously a senior adviser to both Boris Johnson and Michael Gove, Gibbs is another former TaxPayers’ Alliance staffer entering No 10 as a policy expert. He is also in favour of decriminalisation, joining the administration from a policy role at the Centre for Medicinal Cannabis.

The Treasury also announced it would begin a one-year spending review in September, raising the prospect that Johnson wants to make money available for pre-election giveaways in education, health and policing.

Sajid Javid, the chancellor, said it was aimed at giving departments certainty about funding ahead of Brexit on 31 October and delivering on promises of 20,000 extra police officers, additional funding for schools, as well as delivering the government’s pledges on the NHS.

But John McDonnell, the shadow chancellor, said it “smacks of pre-election panic measures by the government”.

“Johnson is splashing a little bit of cash as a publicity stunt, but keeping the door open for even more austerity if a no-deal Brexit breaks the economy,” he said.

The prime minister could face a confidence vote as soon as he comes back from recess in the first week of September, which he may lose if Tory MPs decide to join with opposition parties to bring down the government.

Parliament would have 14 days to find an alternative government, but if that failed, then an election would be triggered. There would be time to hold one before the scheduled Brexit day of 31 October but the law says the date is “on the recommendation of the prime minister”, implying Johnson might be able to choose the date – possibly in the first week of November.

No 10 had no comment on Corbyn’s letter, but a senior Conservative source echoed a statement from Dominic Cummings earlier this week, saying: “No amount of letter writing political stunts will change the fact that politicians don’t get to choose which public votes they respect.”

The Cabinet Office declined to provide clarification of the purdah rules on Thursday but said Sedwill would respond to Corbyn in due course.

Labour is taking no chances and preparing heavily for an election, releasing a highly personal attack advertisement on Thursday that portrayed Johnson as someone who cannot be trusted.

It referenced his sacking by former Tory leader Michael Howard for lying about an affair, and accused him of lying about policy issues such as shutting ticket offices on the London Underground after promising to keep them open.

Labour has selected candidates in 75 target seats but some MPs said they were also concerned about the party’s own readiness and financial situation. Little progress had been made on selections in seats where MPs have defected or plan to retire, though that has been ramped up this month with new candidates being chosen in Birkenhead and Stockport.

Backbench Conservative MPs were highly alarmed about the idea of an election just days after an unpredictable no-deal Brexit, possibly in the midst of food shortages, travel disruption and trade difficulties.

One former cabinet minister said they believed Tory prospects would be dire in a post-Brexit election anyway, because of the under-appreciated threat from remain voters.

“We shed Scotland MPs, demographically die in London, get killed in the south-west by the Lib Dems and lose south-east heartlands because of jobs loss worries. Where is the core? It’s madness, and all predictable,” they said.

Several Tory MPs said they were sceptical about whether No 10 really would pursue such a risky move, and regarded it as a possible ploy to scare would-be rebels away from voting down the government in a confidence motion and triggering an election that could put Corbyn in No 10.

The cross-party group plotting against a no-deal Brexit is already nervous about holding a confidence motion without being completely sure of having the numbers to support an alternative government, in which a compromise candidate such as Ken Clarke or Yvette Cooper would request an extension to article 50. They are increasingly nervous after the Labour frontbench said they would not support an alternative government and the rebels would have to get behind Corbyn as a temporary prime minister if they wanted to stop a no-deal Brexit.

McDonnell quipped on Wednesday that he would send Corbyn in a taxi to Buckingham Palace to see the Queen if Johnson lost a confidence motion and refused to resign.

One Labour MP involved in cross-party discussions said there was a preference among many Tories to try legislative routes for stopping no deal, before going for the more radical option of replacing the government.

Even some pro-Brexit supporters of Johnson were highly alarmed about the idea of an early November election. A senior Tory close to Conservative campaign headquarters said the party would obviously be ready for any eventuality but he was “highly doubtful” that Johnson would risk such a move.