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 it contradicted statements by Mr Johnson in Parliament.
But the prime minister said the UK would still be leaving on 31 October, deal or no deal, “but no delay”.
Talks between the two sides aimed at resolving differences over the Irish border will resume on Monday, with the UK urging the EU “to work with us at pace to agree a new deal”.
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.”
Any extension to the Article 50 process – the mechanism taking the UK out of the EU – would have to be agreed by all 27 other EU leaders.
Mr Johnson has said he would rather be “dead in a ditch” than ask for a delay. In a tweet on Friday afternoon, he said: “New deal or no deal – but no delay.”
The European Commission said its position that the UK’s proposed new deal did “not provide a basis for concluding an agreement” had not changed after a day of talks with UK officials, but discussions would continue on Monday.