Peers voted to back maintaining EU environmental principles and standards by a majority of 50, as they wrapped up weeks of debates over the EU (Withdrawal) Bill, which sets the legal framework for Brexit.
It was the 15th defeat for May on the flagship legislation in the upper chamber, after lawmakers approved various amendments her government opposed. The bill will now go back to the House of Commons, which could reject the various approved amendments.
Pro-Brexit peers were angered by the latest defeat, accusing their colleagues of trying to wreck the legislation.
Conservative lawmaker Michael Lord said their scrutiny had showed the unelected chamber "at its worst".
But other peers said they had simply done their job of scrutinising and revising legislation. The withdrawal bill is crucial to Britain's departure from the European Union, providing for the repeal of the 1972 act that made the country a member and transferring four decades of its regulations onto the British statute books.
Britain adopted the March 29, 2019, date for Brexit when it activated Article 50 of the EU's Lisbon Treaty, setting the clock ticking on a two-year countdown to its departure. The House of Commons passed the landmark bill by a majority of 29 earlier this year after weeks of debate and a damaging government defeat in that chamber. Under parliamentary procedure, the legislation then passed to the House of for scrutiny before returning to the Commons for a final vote.
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