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 law.
Their QC said the submission contradicted statements by the prime minister last week in Parliament.
But Downing Street said the UK would still be leaving the EU on 31 October.
The so-called Benn Act – named after Labour MP Hilary Benn who spearheaded its passage into law – 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.
A senior Downing Street source said: “The government will comply with the Benn Act, which only imposes a very specific narrow duty concerning Parliament’s letter requesting a delay – drafted by an unknown subset of MPs and pro-EU campaigners – and which can be interpreted in different ways.
“But the government is not prevented by the Act from doing other things that cause no delay, including other communications, private and public.
“People will have to wait to see how this is reconciled. The government is making its true position on delay known privately in Europe and this will become public soon.”