UK MPs, including Tories expelled from the party, are preparing legal action in case the PM refuses to seek a delay to Brexit.
A bill requiring Boris Johnson to ask for an extension to the UK’s departure date to avoid a no-deal Brexit on 31 October is set to gain royal assent.
But the PM has said he would “rather be dead in a ditch” than ask for a delay.
Now MPs have lined up a legal team and are willing to go to court to enforce the legislation, if necessary.
The cross-party bill – which requires the prime minister to extend the exit deadline until January unless Parliament agrees a deal with the EU by 19 October – was passed on Friday.
Although the government has said it will abide by the law, Mr Johnson described it as obliging him “in theory” to write to Brussels asking for a “pointless delay”.
The Daily Telegraph reported that the prime minister said seeking another extension is “something I will never do”, fuelling speculation that ministers could try to find a loophole.
The BBC has learned that a cross-party group of MPs have lined up a legal team and that they are prepared, if necessary, to go to court in order to try to compel Mr Johnson to seek a delay.
Downing Street said the British public had been clear that they wanted Brexit done.
David Lidington, Theresa May’s former de facto deputy prime minister, said defying any law sets “a really dangerous precedent”.
He told BBC Radio 4’s Today programme: “The government is bound by the words of any statute that has been duly enacted by the Queen in Parliament, which is a fundamental principle of our constitution and our ministerial code.
“Defying any law sets a really dangerous precedent.
“If you do something in government you need to think: ‘would I be happy if the other lot were in power and they did this to me?’ – and if you wouldn’t be happy with that, then it’s a very good warning signal.”
He added that at a time when other countries were “holding up alternatives to the rule of law and democratic government” it was imperative that British governments always demonstrate they comply with the law.
Mr Lidington resigned as Cabinet office minister in July, in opposition to Mr Johnson’s no-deal Brexit strategy.
This week, however, he supported the government in voting for an early general election.
He said he had been “persuaded” by Boris Johnson, that he “was serious about getting a deal”.
Mr Lidington urged Mr Johnson to “re-double [his] efforts” in talking to a “wide range” of European leaders to get a deal he can bring back to Parliament when prorogation ends on 14 October.
