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 to rule on the extent to which Mr Johnson is bound by the legislation passed by opposition MPs – the “Benn Act” – which requires the government to request an extension to the Brexit deadline if a deal has not been signed off by Parliament by 19 October.