Any attempt by the next prime minister to suspend Parliament to force through a no-deal Brexit is likely to end up in the Supreme Court, a Tory MP has said.
Sir Oliver Letwin told the BBC it was not clear whether MPs had the numbers to stop a no-deal Brexit themselves.
But if the government tried to get its way by by-passing Parliament, he said it would become a “legal question”.
Campaigners say they will challenge any “prorogation” of Parliament to allow Brexit on 31 October without a deal.
As it stands, the UK will leave the EU on Halloween, with or without a legal agreement, unless the next prime minister and other EU leaders agree to another extension.
Boris Johnson, the frontrunner in the race to succeed Theresa May, has ruled this out, saying the UK must leave by that date “do or die”.
However, he has not ruled out potentially suspending – or proroguing – Parliament in the period leading up to 31 October to stop MPs from blocking a no-deal exit.
Opponents of a no-deal Brexit – which would see the UK leave the EU single market and customs union overnight – say it would cause massive economic damage.
Another attempt is currently under way to make it harder for the government to force through no deal against the wishes of Parliament.
Peers are debating amendments to legislation on the re-establishment of devolved government in Northern Ireland, including one stating that MPs and peers should be recalled to debate reports on progress in the autumn if Parliament is closed.
MPs tabled the same amendment in the Commons last week but it was not chosen for debate by Commons Speaker John Bercow.
Backing the move, Lib Dem peer Lord Newby said the only reason peers were contemplating such a “procedural gambit” was to stop the “chicanery” of a possible prorogation.
But former Conservative cabinet minister Lord Forsyth said the issue was a “straw man” being put up by opponents of Brexit.
He said its proponents were ignoring the fact that the Lords and Commons had agreed to the UK leaving the EU when they overwhelmingly passed legislation in 2017 triggering the Article 50 withdrawal process.
Asked earlier about the significance of the latest move, Sir Oliver told BBC Radio 4’s Today that “if you have got a law saying that ministers have got to do things in Parliament on certain dates, then it is at least arguable that ministers cannot prorogue Parliament, thereby preventing themselves from obeying that law”.
If it passed, he suggested it would strengthen MPs’ hand if the whole issue of suspending Parliament ended up going to court.
“I hope the government won’t even try,” he said. “If it takes it into its head to do so at any stage…there will be a Supreme Court challenge.
“The existence of this law would make a (legal) challenge more likely to succeed.”
Anti-Brexit campaigner Gina Miller has said she would seek a judicial review of any attempt to force through a no-deal – a move backed by former PM Sir John Major among others.
She successfully took the government to the Supreme Court in 2017 to force it to get Parliament’s backing to trigger the Article 50 Brexit process.
Sir Oliver said it was “not clear” whether MPs would be able to stop a no-deal exit by legislative means, such as changing the 31 October date, given disagreements between those who wanted a negotiated exit and those who wanted another referendum.
“Can we actually get a parliamentary majority for some action in the autumn if needed to prevent no deal? The answer is we don’t know yet.”
Mr Johnson and his leadership rival Jeremy Hunt will take part in their final head-to-head encounter of the campaign later on Monday, in a debate organised by The Sun.
The result of the leadership contest will be announced on 23 July.