The Prime Minister will follow the terms of the Benn Act, which was passed by opposition MPs last month, lawyers representing the UK Government said during a legal challenge led by SNP MP Joanna Cherry.
The dramatic revelation was made in a submission to the Court of Session in Edinburgh on Friday. The legal action launched at the Outer House of the court seeks to create an order which would force Mr Johnson to send the letter and prohibits him from frustrating the Act’s purpose.
Downing Street refused to comment after extracts from the document were read out in court, but a senior No 10 source told the BBC: “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.”
The Prime Minister has publicly said “we will obey the law, and will come out on October 31” in any event, without specifying how he would achieve the apparently contradictory goals - fuelling speculation that he had identified a loophole to get around the Benn Act.
In a tweet posted after the court ended for the day, Mr Johnson said: “New deal or no deal - but no delay”.
He previously declared he would rather be “dead in a ditch” than ask for a delay.
Any extension to the Article 50 process - the mechanism by which the UK leaves the European Union - would have to be agreed by all 27 other EU leaders.
The legal action - led by businessman Vince Dale, Ms Cherry, and Jolyon Maugham QC - is asking the court to require Mr Johnson to seek an extension to avoid leaving the EU without a deal.
“Boris Johnson simply cannot be trusted to obey the law,” Cherry told reporters.
“The Tory leader has shown time and again that he is dangerous, untrustworthy - and must be removed from office.
”We will continue to press this case to secure a formal binding undertaking to the court that a Brexit extension will be sought - and to ensure that the law is respected in letter and spirit.
“The fact that Downing Street are still briefing they will find a way to avoid an extension, despite Parliament passing a law requiring one, shows how important it is to take every step necessary.
“The Government’s plans for an extreme Brexit would be devastating for Scotland and the UK.
“Whatever happens now, it is clear the opposition parties must unite to bring this Tory government down, secure an extension, prevent a no-deal and hold an election.”
Speaking in court, Andrew Webster QC, representing the UK Government, said the documents it had submitted were a “clear statement” as to what the Prime Minister will do if a deal is not reached by October 19.
He argued there was no need for an order to be made forcing a letter requesting an Article 50 extension to be sent under the terms of the Benn Act, because the court now had it on record it will be sent.
Mr Maugham told reporters that the Prime Minister’s submission said “he would send the letter mandated by the Benn Act” and would not “frustrate” attempts to get an extension.
He said: “That means that it’s impossible for him to say, as he’s been telling Parliament and indeed the rest of us, that we will leave the EU on October 31 come what may.
“That is no longer a true statement - if ever it was a true statement of the law - and the Prime Minister acknowledges that.”
Mr Maugham added: “We want to see the courts tell him that ‘unless you send the letter, no later than October 19, unless you cease trying to frustrate Parliament’s intention, there will be personal consequences for you, you could go to prison’.”
But Eurosceptic MP Steve Baker - the self-styled “Brexit hardman” - insisted that the Prime Minister would still meet the October 31 date.
Mr Baker, leader of the European Research Group of Brexiteer Tories, said: “A source has confirmed that this just means the Government will obey the law but the source confirmed we will leave on October 31.
“It’s not really a development in the position.”
But Mr Baker said he did not know what the Government’s plan was to get around the Benn Act if necessary.
If Mr Johnson - who wants an early election - did request a delay it could play into the hands of the Brexit Party.
The party’s leader, Nigel Farage, said: “Boris said we would leave by October 31 ‘do or die’.
“Why does he keep saying things that are not true?”
Meanwhile, Brexit talks with Brussels on Mr Johnson’s plan to replace the backstop could continue over the weekend.