Boris Johnson will send a letter to the EU asking for a Brexit delay if no deal is agreed by 19 October, according to government papers submitted to a Scottish court.
The document was revealed as campaigners sought a ruling forcing the PM to comply with the Benn Act.
Their QC said the submission contradicted statements by the prime minister last week in Parliament.
Lawyers for the UK government will outline their case later on Friday.
Downing Street refused to comment on the ongoing legal case but stressed again that the UK would be leaving the EU on 31 October.
The legal action has been initiated by businessman Dale Vince, QC Jo Maugham and SNP MP Joanna Cherry.
They want the Court of Session, Scotland’s highest court, to rule on the extent to which Mr Johnson is bound by the legislation passed by opposition MPs – the so-called Benn Act – which requires the government to request an extension to the 31 October Brexit deadline if a deal has not been signed off by Parliament by 19 October.
Mr Johnson has said “we will obey the law, and will come out on 31 October” in any event, without specifying how he would achieve these apparently contradictory goals.
There had been speculation Downing Street had identified a legal loophole to get around the Benn Act, named after Labour’s Hilary Benn who spearheaded the law’s passage through Parliament.
Steve Baker MP, chairman of the European Research Group of Brexiteer Tories, said: “All this means is that government will obey the law.
“It does not mean we will extend. It does not mean we will stay in the EU beyond 31 October. We will leave.”