Peers voted by 335 votes to 244 to ensure parliament – rather than the government – decides the next steps if the prime minister’s exit deal is rejected in the autumn.
Unless the defeat is overturned in the Commons, it increases the chances of softening the deal and sending the government back to the negotiating table if the agreement is rejected.
Earlier, Lord Callanan, a Brexit minister, refused to commit to reversing the vote, saying: “Let’s wait and see what happens – then we will decide our position in the House of Commons.”
The caution may reflect confidence among some Conservatives that there is also enough support in the Commons for the cross-party plan.
The prime minister and other ministers have repeatedly insisted that rejection of her deal will simply mean the UK leaving the EU, in March next year, with no agreement at all.
For that reason, Sir Keir Starmer Labour’s shadow Brexit secretary, hailed the thumping defeat as “a hugely significant moment in the fight to ensure parliament has a proper role”.
“If parliament votes down the Article 50 deal, then parliament must decide what happens next, Sir Keir said.
“Under no circumstances can the prime minister be given a blank cheque to crash the UK out of the EU without a deal.
“I would urge the prime minister to accept this cross-party amendment and recognise that there is no majority in parliament or the country for a no deal Brexit.”
Viscount Hailsham, the former Tory minister who tabled the amendment, said it was aimed at ensuring “that the future of our country is determined by parliament and not by ministers”.
“In democracies, both parliament and the electorate have the right to change their minds. Unchangeable decisions have no place in a democracy,” he added.
And Caroline Lucas, the co-leader of the Green party and supporter of the People’s Vote campaign, said it raised the prospects of a fresh referendum on the exit deal.
“MPs should respond by ensuring the public are also able to express their view on the final Brexit deal, which will affect generations for generations to come,” she said.
A total of 19 Conservative peers defied the government to back the amendment, including former deputy prime minister Lord Heseltine and former cabinet ministers Lord Patten and Lord Willetts.
It is the second crushing defeat inflicted by the Lords on the main Brexit bill, after last week’s vote to keep the UK in a customs union after departure.
The Brexit department said it was “disappointed” by the vote – but again stepped short of committing to reversing it.
“What this amendment would do is weaken the UK’s hand in our negotiations with the EU by giving parliament unprecedented powers to instruct the government to do anything with regard to the negotiations – including trying to keep the UK in the EU indefinitely,” it warned.
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