Boris Johnson will send a letter to the EU requesting a Brexit delay if no agreement is reached by 19 October
The document was uncovered when activists sought a decision that forced the prime minister to comply with the Benn Act.
Their quality control said the filing contradicted the Prime Minister's statements in Parliament last week.
UK Government lawyers will explain their case later on Friday.
Downing Street refused to comment on the ongoing litigation, but reiterated that Britain would leave the EU on 31
The lawsuit has been initiated by businessman Dale Vince, QC Jo Maugham and SNP Deputy Joanna Cherry
They want the Court of Session, Scotland's highest court, to decide to what extent Mr. Johnson has been appointed by the Members of the opposition adopted laws is bound – the so-called Benn Act – according to which the government must apply for an extension of the Brexit deadline for October 31, when a deal was not signed by Parliament until 19 October.
Mr. Johnson said in any case, "We will obey the law and come out on October 31," without saying exactly how he would do to achieve these seemingly contradictory goals.
There had been speculation that Downing Street had found a loophole to circumvent the Benn Act, named after Hilary Benn of Labor, who led the parliament passing the bill.
Steve Baker MP, chairman of the European Research Group of Brexiteer Tories, said, "This just means that the government will comply with the law."
"That does not mean that we will extend it. This does not mean that we will stay in the EU after 31 October. We will go. "