Jeremy Corbyn is urging voters to back him with a pledge to “get Brexit sorted” in six months if he becomes prime minister.
The Labour leader will outline his plans for renegotiating a new deal based on a customs union with the European Union – and a referendum offering the option to vote for that agreement or Remain – in a major speech on Tuesday.
But Boris Johnson accused him of failing to honour the result of the 2016 referendum, and said the plan means “2020 will be lost to more dither and delay over Brexit”.
It comes as parliament prepares to shut down on Tuesday evening in the run-up to the general election on Thursday 12 December.
MPs are expected to use their final day to hear speeches from those choosing not to stand again, officiated by the newly elected Commons Speaker Sir Lindsay Hoyle.
Away from Westminster, Mr Corbyn will address activists in Essex, and accuse the government of trying to tear apart food standards in the quest for a post-Brexit trade deal with the US.
He will claim: “Boris Johnson’s Conservatives want to hijack Brexit to sell out the NHS and sell out working people by stripping away their rights.
“Given the chance, they’ll run down our rights at work our entitlements to holidays, breaks and leave.
“Given the chance, they’ll slash food standards to US levels where ‘acceptable levels’ of rat hairs in paprika and maggots in orange juice are allowed, and they’ll put chlorinated chicken on our supermarket shelves.
“And given the chance, they’ll water down the rules on air pollution and our environment that keep us safe.”
But, in a letter to the Labour leader, Mr Johnson hit back – accusing him of frustrating Brexit by trying to avoid no-deal and voting down the three-day timetable to pass an EU divorce deal.
“Your current position seems to be that you want to go back to square one,” the prime minister wrote.
“You want to throw out the great new deal we have reached with our European friends, and instead negotiate a whole new treaty from scratch.
“Even assuming the EU agrees to go back to the very beginning, this will take months and possibly years to do.”
Mr Johnson added that, if the Conservatives win a majority next month, he will ensure Brexit happens “by the end of January” – the current deadline set by Brussels.
Their clash comes 38 days before the election, called to break a parliamentary deadlock over Brexit.
Liberal Democrat leader Jo Swinson is standing on a platform to cancel Brexit if she becomes prime minister.
She did not rule out a report that it could see an alliance across up to 60 seats in the 12 December election, when speaking to Sky News’ Sophy Ridge on Sunday.
“The specifics of announcements will be made in due course but it’s well understood that these discussions have been taking place,” Ms Swinson explained.
The Brexit Election: For the fastest results service and in-depth analysis watch Sky News live from 10pm on Thursday 12 December, with a KayBurley@Breakfast election special on Friday 13 December