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Peers have handed Downing Street a second defeat in the space of a week, dealing a setback to Theresa May’s Brexit strategy.
The House of Lords voted 366-268 to back a requirement for ministers to bring the final terms of Britain’s exit from the EU back to parliament for approval, securing a majority of 98 to clear the measure despite Mrs May’s warning that doing so would undermine her negotiating position and risk a “bad Brexit”.
634 peers voted, the largest number in a vote since 1999.
A succession of Tory peers spoke out against the prime minister.
Former MP Lord Cormack urged peers to “send a message” to Mrs May and “vote to put Parliament in its rightful place”.
Lord Heseltine – the former deputy prime minister – said that those who had voted Remain in last year’s referendum “have a right to be heard” and “the fightback starts here”.
The amendment “secures in law the government’s commitment to ensure parliament is the custodian of our national sovereignty”, he said.
Viscount Hailsham, a former minister, said the amendment reflected that “it is the parliament and not the executive which should be the final arbiter of our country’s future”.
They faced opposition from fellow Conservatives, including eurosceptic former chancellor Lord Lawson, who accused his rebellious colleagues of wanting to “have their cake and eat it”. Giving parliament the power to prevent Brexit would be “an unconscionable rejection of the referendum result”, which could trigger a constitutional crisis, he said.
Lord Bridges, a minister in the Exiting the EU department, said the amendment was “unnecessary, damaging to the national interest and may be used to block the intention of the British people”.
The addition to the legislation gives MPs and peers a vote on the terms of Britain’s exit early enough for parliament to insist that Mrs May return to Brussels to seek better terms.
By contrast, Mrs May wants parliament to be offered a “take it or leave it” vote to rubber-stamp a deal before it is put before the European Parliament. If MPs reject the deal, the UK would then be left to trade with the EU under the tariff regime of the World Trade Organisation.
The House of Lords also defied the prime minister last week, voting by an overwhelming majority to back an amendment to the government’s Brexit legislation that would guarantee the rights of EU citizens living in the UK.
The bill – including the two amendments – now returns to the Commons, where Downing Street is confident it has the numbers to strike down the changes.
Mrs May is likely to have the support of the Democratic Unionist party, which fields eight MPs, while a handful of Eurosceptic Labour MPs could back her.
The prime minister has a working majority of just 17 and faces a potential rebellion by pro-EU Conservatives.
Opposition aides estimated that about 20 Tory MPs were considering supporting the amendments, but they acknowledged that many of them were likely to duck a direct confrontation with government whips.