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.