The Labour leader, Jeremy Corbyn, has written to the cabinet secretary expressing fears that Boris Johnson intends taking the UK out of the EU without a deal even if the Prime Minister loses a no confidence motion in parliament.

Corbyn wants Sir Mark Sedwell, Britain’s most senior civil servant, to stop such an outcome.

There are reports that Boris could seek to hang on long enough to ensure Britain is out of the EU before going to the polls if he is defeated in a vote of confidence when MPs return from their summer break in September.

As it stands, under the latest extension to the Article 50 withdrawal process by the EU, Britain is due to leave on October 31. In his letter to Sedwell, Corbyn said such a course of action would be “unprecedented” and “unconstitutional”.

He asked the cabinet secretary to confirm that if the UK is due to leave the EU without a deal while an election is under way, the government should seek another time-limited extension to Article 50 to allow the voters to decide.

He wrote: “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.”

Speaking for Labour, the shadow transport secretary Andy McDonald told BBC Radio 4’s Today programme on Friday: “When the Prime Minister of the day is intent on departing from protocols, custom and practice that has served us well over many hundreds of years, we do need to have some voice of sanity.”

“If it cannot come from the politician and immediate adviser to the Prime Minister, then we have no other choice than to go to the head of the civil service and say, ‘You’re the custodian of the civil service, you’ve got a role to play here and we do expect you to intervene and guide and advise.’ ”

McDonald said Labour is committed to putting “whatever deal is negotiated back to the people”.

Pressed if Corbyn would be leading a government which wants to remain in the EU or negotiating to leave, McDonald said: “We would be negotiating a deal and we could go down these hypothetical rabbit holes forever.”

However, Brexiteers argue parliament has already voted to leave the EU, agreeing to trigger the Article 50 process and passing legislation setting Britain’s withdrawal date for October 31.

Officials said that Sedwell would be replying to Corbyn, but senior Tories dismissed the Labour leader’s letter as a “political stunt”.

A senior Conservative source said: “Jeremy Corbyn will do anything to get his hand on the keys to number 10. No amount of letter-writing political stunts will change the fact that politicians don’t get to choose which public votes they respect.”

Meanwhile, Boris has also sought to address concerns over access to the UK after Brexit, announcing a new fast-track visa route which will be created for foreign scientists coming to the UK.

Unveiling the plans on a visit to the Culham Science Centre in Oxfordshire on Thursday, Boris said he wanted to ensure the country’s immigration system attracted “the very best minds from around the world”.

He said: “Britain has a proud history of innovation, with home-grown inventions spanning from the humble bicycle to the light bulb.

“We were home to the world's first national DNA database, we discovered graphene, and our cutting-edge scientists should be proud to follow in the footsteps of titans like Ada Lovelace and Nobel Laureates Francis Crick and Peter Higgs.

“But to ensure we continue to lead the way in the advancement of knowledge, we have to not only support the talent that we already have here, but also ensure our immigration system attracts the very best minds from around the world.”

Boris was responding, in part, to concern about the consequences of Brexit and especially a no deal Brexit expressed by Sir Venkatraman Ramakrishnan, the president of the Royal Society.