In the aftermath of the stunning Tory general election win, the government has amended its Brexit bill to “legally prohibit” any further extension.
The withdrawal agreement bill (WAB) is due to come before the Commons this week.
Under current plans, Johnson intends to end Britain’s EU membership on January 31, with a transition period to run to the end of 2020 while the government negotiates a free trade deal with Brussels.
However, key EU figures – including chief negotiator Michel Barnier – have expressed scepticism that a deal can be agreed in time, raising the fresh prospect of a no-deal break unless there is an extension.
A No 10 source said: “Last week the public voted for a government that would get Brexit done and move this country forward – and that’s exactly what we intend to do starting this week.
“Our manifesto made clear that we will not extend the implementation period and the new withdrawal agreement bill will legally prohibit government agreeing to any extension.”
The prime minister repeatedly promised during the election campaign that he would not seek any extension to the transition period.
The commitment was instrumental in persuading Nigel Farage not to stand Brexit Party candidates in Conservative held seats.
However, after Johnson was returned with an 80 majority, there was speculation that he could use his strengthened position to seek an extension if more time was needed to get a trade deal.
The latest move would appear to have put paid to that.
Johnson meanwhile has been accused of showing “two fingers to democracy” after announcing Nicky Morgan will carry on as culture secretary, despite her quitting the Commons.
No 10 said the former MP for Loughborough would be made a life peer and would answer questions in the House of Lords.
There were signs that her appointment may only be temporary pending a full-scale Cabinet reshuffle expected in February.
But it still drew a furious response from opposition MPs, with former shadow culture secretary Chris Bryant saying it “stinks”.
“You abandon your constituents, eschew the tough work of representing a constituency but remain in the Cabinet. That really is two fingers up to democracy,” he said.