The House of Lords dealt the Prime Minister a blow tonight demanding that Britain join a customs union on the day it leaves the EU.

The amendment, which was supported by Labour , Lib Dems and peers from across the house, will force the Prime Minister to schedule another vote in the Commons if she wants to maintain her current policy of leaving the customs union when we leave the EU.

It also signals Labour moving to put in place arrangements for a softer Brexit deal, with the amendment being moved with the support of the party's leadership.

Peers backed the vote 207 for -141 against.

Labour said "ministers must now drop their red lines on Brexit" and "embark on a fresh approach" to guarantee access to the a customs union.

Now Mrs May could be forced to squeeze another vote into a packed schedule ahead of 'Exit Day' on March 29th.

(Image: Getty)

This would give MPs a chance to double down on the Lord's decision - leading to a softer break with the EU than those outlined in Mrs Mays deal.

Theresa May would have no guarantee of winning the vote and an increasingly fractured parliament.

But MPs have already rejected amendments calling for the UK to remain in the customs union.

(Image: AFP/Getty Images)

The successful amendment read: "It shall be the objective of Her Majesty's Government to take all necessary steps to implement an international trade agreement which enables the United Kingdom to participate after exit day in a customs union with the European Union."

The customs union is an agreement between the members of the EU which allows goods to move from one country to the other without inspection or having to pay tariffs.

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Lord Stevenson of Balmacara, Shadow International Trade Minister said: “The success of this cross-party Lords motion gives both the government and the House of Commons a chance to step back from the disaster of a No Deal, and to deliver an outcome which would satisfy the clear majority of people in the UK.

“Ministers must now drop their red lines on Brexit and embark on a fresh approach to the negotiations with the EU – based around a new Customs Union that protects jobs, secures opportunities for our industries, and removes the need for a hard border in Ireland.”