British 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.
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, the BBC reported.
Downing Street said the British public had been clear that they wanted Brexit done.
What does the no-deal bill say?
The bill, presented by Labour MP Hilary Benn, says the prime minister will have until 19 October to either pass a deal in Parliament or get MPs to approve a no-deal Brexit.
Once this deadline has passed, he will have to request an extension to the UK's departure date to 31 January 2020.
Unusually, the bill stipulates the wording of the letter Mr Johnson would have to write to the president of the European Council.
If the EU responds by proposing a different date, the PM will have two days to accept that proposal.
During that time, MPs - not the government - will have the opportunity to reject that date.
The bill also requires ministers to report to the House of Commons over the next few months. potentially providing more opportunities to take control of the timetable.
Meanwhile, a number of cabinet sources have told the BBC in recent days that they have significant concerns about Number 10's strategy.
It comes in the wake of a series of Parliamentary defeats for the government, beginning after Mr Johnson announced his decision to suspend Parliament for five weeks in September and October.
First, the prime minister lost control of the House of Commons agenda.
That allowed opposition MPs and rebel Tories to put forward the bill to prevent a no-deal Brexit, which Mr Johnson said "scuppered" his negotiations with the EU.
In response, the prime minister expelled 21 of his own MPs for rebelling against the government over the vote and then called for a general election.
But on Friday, Labour, the Liberal Democrats, SNP and Plaid Cymru jointly agreed to reject Mr Johnson's demand for a snap poll before the EU summit in mid-October.
The day before, the prime minister's younger brother, Jo Johnson, resigned as an MP and minister, saying he was "torn between family loyalty and the national interest".
According to the Daily Telegraph, Mr Johnson wrote to Conservative Party members on Friday night, saying Labour MPs had "left us no choice" but to call for an election.
He said: "They just passed a law that would force me to beg Brussels for an extension to the Brexit deadline. This is something I will never do."
No 10 said an election would allow the public to choose between the government's approach - Mr Johnson's commitment to leave on 31 October, either with a renegotiated deal or no deal - and "more delay, more dither" from Labour.
But opposition MPs say they will only agree to an election when the extension to the Brexit deadline has been secured, to ensure the UK does not "crash out" without a deal.