Number 10 is yet to spell out how Mr Johnson plans not to ask the EU to delay Brexit to avoid no-deal without breaking the law.
Opposition MPs passed a bill earlier this month forcing Mr Johnson to ask Brussels for a delay if he has not passed a deal in parliament by 19 October.
The government has not explained how it will get round the legislation, but Foreign Secretary Dominic Raab has said it will “test” the law to the “limit”.
He dismissed criticism about the lack of detail made public on proposals being made to the EU.
“This is not about whether enough technical content has been provided but about whether there is the political will,” he told Sky News on Monday.
The matter is expected to end up in the Supreme Court starting on Tuesday.