The prime minister, who has struggled to unite her Conservatives over Brexit, has said Britain will leave the EU's single market and customs union after it quits the bloc next March so that London can negotiate its own free trade deals.
The upper house of Parliament resumes debate on Theresa May's legislation to leave the European Union today, with a cross-party alliance of MPs eager to reduce any potential damage that could be caused.
It is simply impossible to do Brexit without a customs union so the House of Lords has spoken up for good and responsible government.
May's government has already postponed votes on a trade bill and a taxation bill after lawmakers attempted to force through amendments that would instruct ministers to try to stay in a customs union with the EU.
Peers voted in favour of the amendment to the flagship EU Withdrawal Bill by 348 votes to 225, a majority of 123.
Shadow Brexit Secretary and Labour MP Sir Keir Starmer said it was a step forward for those arguing that the United Kingdom ought to stay in a customs union with the EU.
Britain's House of Lords inflicted an embarrassing defeat on Theresa May's government on Wednesday, challenging her refusal to remain in a customs union with the European Union after Brexit.
Other battlegrounds in the Lords include measures to limit the Government's ability to use so-called Henry VIII powers, which restrict Parliament's ability to scrutinise future legislative changes required as a result of Brexit.
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At the very least, it will add to the pressure on her to execute a U-turn and stay in a customs union.
"The fundamental goal of this Bill is to prepare our statute book for exit day, it is not about the terms of our exit".
It's still not clear how the government would react to a defeat, and to what extent the amendments will force May to change her policy.
The opposition Labour party has argued at different times that Britain must remain in the Customs' Union to preserve the economic unity and peace in Ireland. "We won't do better than the customs union".
Political commentators say May's government could face further defeats as members of the House of Lords go line-by-line through the Brexit bill.
"Theresa May must now listen to the growing chorus of voices who are urging her to drop her redline on a customs union and rethink her approach".
"As the PM has set out, that means we are leaving the customs union", the spokesman said. In a speech in September, she ruled out staying in the customs union.
They also voted 314 votes to 217 by approving amendment 11, which aims to protect people's rights. "I'm pretty optimistic that unless the government makes some meaningful concessions on some of them that we should get them through". "I don't think blithering on about Global Britain or pretending we haven't been Global Britain for years, or repeating the Road to Mandalay whenever one is travelling, is going to make a difference to trading opportunities", he said.