Britain's House of Lords inflicted another defeat on the government Monday over its flagship Brexit bill, sending it back to MPs and setting up a fresh showdown between Prime Minister Theresa May and her pro-European rebels.
Unelected peers in the upper house voted by 354 to 235 to support a rebel amendment on the role parliament should play if the government fails to secure a deal with the European Union before Britain leaves the bloc in March 2019.
"I want to ensure that parliament does have a meaningful vote and I don't want to see that left to chance," said Lord Hailsham, the member of May's Conservative Party who proposed the motion.
The amendment to the EU (Withdrawal) Bill was drawn up in consultation with pro-European MPs in the lower House of Commons, who will have a chance to vote on it themselves on Wednesday.
They had threatened to rebel on the same issue when they debated the bill last week, but held off following personal assurances from May that she would heed their concerns.
However, her compromise amendment fell short of their expectations, and peers agreed to back an alternative so the MPs could vote again when the bill returns to them, in a process known as "ping-pong".
May earlier warned that any attempt by parliament to take control of the Brexit negotiations would weaken her hand.
"Of course we have been listening to concerns about the role of parliament," she told reporters.
"But we need to make sure that parliament can't tie the government's hands in negotiation and can't overturn the will of the British people."
Despite the stuttering progress in the talks with Brussels, both sides still hope to reach a deal in October.
The government has promised lawmakers a vote on the final deal, but the issue at stake is what happens if they reject it.
Pro-Europeans want to ensure there is some way of holding the government to account in what would be a crisis situation.
The EU (Withdrawal) Bill would formally end Britain's membership of the bloc and transfer more than 40 years of European law on to the British statute books.
May is on a tightrope as her Conservative minority government relies on the backing of 10 MPs from Northern Ireland's Democratic Unionist Party for a slim majority in the 650-seat elected Commons chamber.
Dominic Grieve, the former attorney general who heads up the pro-European faction, told BBC television that a future vote on a Brexit deal could see May tumble.
"We could collapse the government, and I assure you I wake up at 2:00 am in a cold sweat thinking about the problems that we have put on our shoulders," he said.
An added risk for the rebels is that if May does fall, it could open the door for an arch-Brexiteer to take over.
May risked stirring the pot further on Monday by announcing new money for the state-funded National Health Service (NHS) based on a "Brexit dividend".
She said part of the £20 billion ($27 billion, 23 billion euros) injection would be funded by "the money we no longer spend on our annual membership subscription to the European Union".
The promise to divert money from the EU to the NHS was a key tenet of the pro-Brexit campaign in the 2016 referendum, but highly controversial.
Independent experts warn the figures do not add up, arguing the economy is already slowing as a result of Brexit, which will cost far more than membership fees.
May admitted some of the money would be funded by taxation, risking provoking the Brexit-supporting right wing of her party, who favour reducing public spending.