THERESA May has been accused of siding with “cynical Brexiteers” over her refusal to pursue a customs union with the European Union.

SNP Westminster leader Ian Blackford told the Prime Minister she and her government were “not working in the interest” of the British people with her approach to Brexit.

“The CBI, the NFU, the Scottish Government, the Welsh Government, the House of Lords and overwhelmingly members of this House want the UK to remain in the customs union,” he said.

“Why is the Prime Minister on the side of her cynical Brexiteers? Frontbenchers are not working in the interest of all nations of the United Kingdom.”

The PM is under pressure to soften her opposition to a customs union after the EU’s chief negotiator Michel Barnier last week rejected her proposals on keeping a soft border in Ireland.

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There is also increasing concern among businesses about the economic impact of leaving he customs union amid an absence of progress on trade talks with countries outside the EU.

But the PM responded: “The British people voted to leave the European Union, in voting to leave the European Union they voted to leave the single market and the customs union.

“What we want to ensure is that we as a country are able to independently negotiate free trade deals around the rest of the world, that we are also able to ensure that we deliver on our commitment for no hard border between Northern Ireland and Ireland and that we have as frictionless a border as possible between the United Kingdom and the European Union.

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“What businesses tell me is that they want that tariff-free, frictionless border and that’s what we’re negotiating for them.”

A symbolic vote today on backing remaining in the customs union could see May face a further defeat.

According to reports, the Prime Minister has told ministers to stay away from the debate in the Commons.

It is one of a series of critical votes in the next few months and could pave the way for a dramatic loss when MPs vote on an amendment to the Brexit bill that could force ministers to abandon their opposition to a customs union.

Allies of May have reportedly said that failing to confront demands for a change of course by pro-European Tories would only delay the Prime Minister’s problem.

Meanwhile, the Government suffered another defeat in the Lords over flagship Brexit legislation as peers backed stricter controls on ministerial regulation-making powers.

Voting was 349 to 221 – a majority of 128 – for a cross-party amendment to the European Union (Withdrawal) Bill.

It was the sixth defeat inflicted by peers since the bill’s report stage began just a week ago.

This time the issue was so-called Henry VIII powers which allow ministers to amend EU rules and regulations when they are transferred on to the UK statute book post-Brexit, with what critics claim is little parliamentary scrutiny.

Spearheading the change to the bill, former Commons clerk Lord Lisvane said it currently contained “heavyweight” powers for ministers to make regulations subject to an “inadequate” subjective test.

He said the issue was about the “balance of power” between ministers and Parliament.

“Whichever side of the Brexit argument they stand, people might reasonably believe that ‘taking back control’ would be under the sovereignty of Parliament rather than ceding swathes of power to the executive,” he added.

Conservative Lord Cormack, backing the amendment, said the powers of ministers should be “contained in a sensible and democratic manner by Parliament”.