The House of Lords has backed an attempt to prevent a future prime minister suspending Parliament to push through a no-deal Brexit.
The measure will now go to MPs for a vote on Thursday, after peers defeated the government by 272 votes to 169.
Tory leadership frontrunner Boris Johnson has not ruled out suspending Parliament to ensure the UK could leave by 31 October, even without a deal.
Labour said suspension would be “constitutionally improper”.
Mr Johnson’s leadership rival, Jeremy Hunt, has ruled it out.
If the 31 October deadline is reached without a deal being agreed, the UK will leave the EU without one.
MPs have consistently voted against this option, but the prime minister could try to get around that by closing Parliament – proroguing – in the run-up to Brexit day, denying them an opportunity to block it.
The Lords cross-party measure to prevent a suspension came in the form of an amendment to a bill on restoring devolved government in Northern Ireland.
Last week, MPs amended the bill to require ministers to give fortnightly reports to Parliament throughout October on progress to restore devolution.
The hope of those behind that amendment was that it would make it more difficult for Parliament to be shut down.
- Can the new prime minister force a no-deal Brexit?
- Brexit battle could end up in court, warns Letwin
The latest move by peers is designed to strengthen that position, by making sure the fortnightly reports would have to be physically debated in the Commons and therefore it could not be suspended.
The amendment will have to be approved by MPs on Thursday in order to make it into the final version of the bill.
Former independent reviewer of terrorism legislation Lord Anderson said it would require Parliament to sit at specified intervals between September and December.
Also backing the move, Labour peer Lord Goldsmith said suspending – or proroguing – Parliament to push through no deal would be “a very bad idea”.
“It’s Parliament who ensures we remain a free land – that is how we do our democracy. To allow that to be set aside would be wrong,” he added.
Government minister Lord Duncan of Springbank opposed the amendment, arguing it would send a message that peers can “use Northern Ireland for different purposes when we choose to do so”.
Also opposing the move, Conservative peer Lord True said Mr Johnson had “never said” he would suspend Parliament, and “outrage” about the possibility of this occurring has been “got up” by “Remainers”.
Former Tory prime minister John Major has said he would seek a judicial review in the courts if the new prime minister tried to suspend Parliament.
Campaigner Gina Miller has threatened the same action.
Tory MP Sir Oliver Letwin, who opposes a no-deal Brexit, has warned that any attempt to do so is likely to end up in the Supreme Court.