Jeremy Corbyn has urged a top civil servant to block Boris Johnson from pushing through a no-deal Brexit if an election is called.
There are reports Mr Johnson 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 possible vote of confidence when MPs return in September.
If the Prime Minister loses, he would have two weeks to win a second vote before a general election is called – during which time, a no-deal Brexit could go ahead.
But in a letter to Cabinet Secretary Sir Mark Sedwill, the Labour leader said the move would be an "unprecedented, unconstitutional and anti-democratic abuse of power".
Mr Corbyn asked the senior civil servant to confirm that if the UK is to leave the EU without a deal while an election is underway, the government should seek another 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.”
But Brexiteers argue that Parliament has voted to leave the EU, and not delivering a withdrawal would deny democracy.
Senior Tories have dismissed the letter as a “political stunt”, with one saying “Jeremy Corbyn will do anything to get his hands on the keys to Number 10”.
Officials have said that Sir Mark would be replying to Mr Corbyn.