The UK government has promised to give MPs and Lords a "meaningful vote" on the final withdrawal deal, in which they'll vote on whether to approve or reject the terms of the deal negotiated between UK and European Union negotiators.
Peers backed an amendment to the European Union (Withdrawal) Bill by 335 to 244, a majority of 91, to give parliament a decisive say on the outcome of the Brexit negotiations, including in the event of a no-deal scenario.
Peer Conservative Viscount Hailsham, told Lords the principle of parliamentary sovereignty was "fundamental to our liberties and must not be betrayed" But ex Tory leader Lord Howard said the idea of giving Parliament a veto over Brexit, voted by the public in a referendum, was "fundamentally misconceived".
The amendment, known as Clause 49, proposes that Parliament can determine the Government's course of action if the Commons rejects. It would force May to go against her own parliament if she disagreed with their vote, setting up a conflict that would be nearly impossible for her to resolve.
Both sides hope to negotiate the UK's withdrawal agreement by this October in order to give the United Kingdom and European Parliaments enough time to debate and vote on it before the moment of departure.
"Labour won the argument at the end of a year ago for Parliament to be given a meaningful vote on the terms of our withdrawal from the EU".
"If Parliament votes down the Article 50 deal, then Parliament must decide what happens next".
It could open the way for parliament to back staying in the EU's customs union and single market -something now ruled out by the prime minister.
"Second, it demands specific votes by artificial deadlines which, if not met, would give parliament the power to micromanage the government on how to carry out these negotiations". This would give MPs and Lords four months to decide what to do next before Britain's scheduled exit date of March 2019. Last week Brexit Secretary David Davis said the motion to be considered by MPs on the final deal would be amendable - raising the prospect of MPs having greater influence over the process.
"The rules are clear". MPs have already defeated the government once on the issue of a meaningful vote and the issue will now return to the Commons for it to be decided once and for all.
"The stakes would be high. So at this stage, we should not close off any options for what might happen - more time to negotiate, a pause in the process or anything else".
The move was supported by 19 Conservative peers who rebelled against their party.
After the vote, Brexit Minister Lord Callanan said the amendment would weaken the government's negotiating hand with Brussels. However, the cabinet's Brexiters are preparing a big push this week against any suggestion Britain should remain inside the customs union, which would make it impossible for the United Kingdom to negotiate its own trade deals.
Britain's unelected House of Lords delivered a crushing Brexit blow for Prime Minister Theresa May Monday night.