Theresa May has reportedly secured concessions from Brussels to keep the whole of the UK in a customs union in the wake of Britain’s withdrawal from the European Union.

Reaching an agreement would prevent the need for Northern Ireland to be treated differently from the rest of the UK, a main stumbling block during Brexit negotiations.

The “secret” deal would avoid the need for an Irish backstop and would be written into the legally binding deal, according to The Sunday Times.

However, Downing Street has poured cold water on the report, calling it speculation.

The EU has reportedly suggested a backstop post-Brexit customs arrangement covering all of the UK could give mainland Britain some scope to set trade rules.

Preparations for a final deal were far more advanced than previously disclosed, the report said, and would lead to a document of 50 pages or more being published.

The agreement would include an “exit clause” designed to convince Brexit-supporting MPs that remaining in the customs union was only temporary, The Sunday Times said.

Ms May’s cabinet will meet on Tuesday to discuss her plan, and she hopes there will be enough progress by Friday for the EU to announce a special summit, the newspaper reported.

The prime minister’s office described the report as speculation but claimed most of a deal on Britain’s exit from the bloc in March 2019 had been agreed.

“This is all speculation,” a spokesman for Ms May said. “The prime minister has been clear that we are making good progress on the future relationship, and 95 per cent of the withdrawal agreement is now settled and negotiations are ongoing.”

What does a no-deal Brexit mean?

Meanwhile, more than 70 business figures are calling for a public vote on the final terms of Britain’s exit from the European Union, warning that the country faces “either a blindfold or a destructive hard Brexit”, the newspaper also reported.

Companies are increasingly concerned about the prospect of Britain crashing out of the EU without an agreement, or fear that politicians will sign up to a deal that limits companies’ access to the continent’s markets.

A new group called Business for a People’s Vote, which includes Justin King, former boss Sainsbury’s, and John Neill, head of the car parts supplier Unipart, is to be launched as campaigners raise the pressure on politicians.

“We are now facing either a blindfold or a destructive hard Brexit. Both these options will further depress investment,” the group said in a letter.

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“They will be bad for business and bad for working people. Given that neither was on the ballot in 2016, we believe the ultimate choice should be handed back to the public with a People’s Vote.”

The government’s Brexit department said it was confident of securing a deal that works for businesses, and reiterated its opposition to a second referendum.

“The people of the United Kingdom have already had their say in one of the biggest democratic exercises this country has ever seen, and the prime minister has made it clear that there is not going to be a second referendum,” a spokesman said.