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Pressure is mounting on Theresa May to keep Britain in a customs union after the Government lost a symbolic vote in the House of Commons.

Pro-Remain Tories warned their party would "not be forgiven for a generation" if the party ignores the evidence for peace in Northern Ireland.

But many prominent anti-EU campaigners did not show up to a parliamentary debate on urging the government to press for the negotiation of "an effective customs union" with the bloc after Brexit.

It comes after Home Secretary Amber Rudd was forced to issue a swift correction after suggesting the cabinet's view on the Customs Union was still being decided - despite official Tory policy being to leave it.

Conservative former cabinet minister Nicky Morgan told the House of Commons: "This country has been asked to experiment at other people's pleasure with a free trade policy where we do not know what the costs will be for constituents and businesses in this country.

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"I say to my party: if we undermine and ignore the evidence for peace in Northern Ireland, and we undermine the business and financial security of the people in this country, we will not be forgiven for a generation."

While Tory former minister Ed Vaizey said: "It seems to me if you're going to leave the European Union, which you are, and you're a free and sovereign nation, you can then make decisions that are in the interests of your economy.

"If it's in the interests of our economy to be a member of a customs union, it should be possible for Parliament to debate that and make that decision without being accused of betraying the will of the British people."

The House of Commons approved the motion on a customs union verbally without a vote in a procedure that does not require the government to take any action.

The, or a, customs union is one of the main flashpoints in the debate over Britain's impending exit from the EU and Brexit campaigners are sensitive to any suggestion that Britain might stay in such an association with the bloc.

That was brought home when Amber Rudd was forced to clarify her remarks at a lunch with journalists that had seemed to suggest the government was still working out its policy on the customs union.

(Image: AFP)

She later again said that the government's position was to leave the customs union, which sets external tariffs for goods imported into the bloc, so that Britain can independently negotiate trade deals with other countries.

But Downing Street later contradicted the Home Secretary.

May’s spokesman said: “The government is absolutely clear and without ambiguity that we are leaving the customs union and not joining a customs union.

“It’s the position of the prime minister, the cabinet and the entire government that we will be leaving the customs union and be free to sign our own trade deals around the world.”

Number 10 said there was “a discussion around the issue of which two customs options the government opts for - but are we leaving the customs union? The answer is categorically yes.”

Brexit supporters say that remaining in any EU grouping will reduce Britain to a "vassal state".

Remainers have been emboldened after Britain's upper house of parliament, the House of Lords, also challenged her adherence to what some of her critics say is an unnecessary red line - one which may mean the return of a hard border with EU member Ireland.

The EU has added its weight to the argument, with diplomats and officials saying they would offer Britain a closer relationship if it stays in the customs union, which would help break the latest stalemate in talks.

But May and her ministers have stuck to their position, forced to repeat their mantra almost daily that Britain will be "leaving the customs union and will be free to strike our own trade deals around the world".