The DUP has joined pro-Brexit Conservatives who are threatening a rebellion against Theresa May by tabling amendments in the Commons aiming to obstruct the Cabinet's Brexit proposals.

It comes as the Prime Minister already faces intensifying pressure from the pro-Brexit wing of her party to drop her Chequers agreement on the future UK-EU relationship - a plan that has led to the resignations of Cabinet heavyweights Boris Johnson and David Davis.

Jacob Rees-Mogg, who chairs the European Research Group (ERG) of Tory Eurosceptics, has laid down four amendments to the Government's Customs Bill which MPs could be asked to vote on next week.

The amendments were lodged yesterday and Priti Patel, the former Cabinet minister, has signed them, alongside MPs Marcus Fysh, John Redwood, Bernard Jenkin and Craig McKinlay.

It is highly unlikely Labour would support Mr Rees-Mogg and the Brexiteers on the Tory backbenches, but it would be the first significant test of strength for her Brexit critics.

One of the amendments if passed will prohibit the UK collecting tax or duties on behalf of another territory "without reciprocity".

This would cause a serious headache for Downing Street as it would block the current proposal of a "facilitated customs arrangement" which aims to collect taxes after Brexit - on behalf of the EU - for goods passing through the UK en route to the continent.

A second amendment - signed by the East Antrim DUP MP Sammy Wilson, former Northern Ireland Secretary Owen Paterson and Labour's Kate Hoey - orders the Government to commit itself in law not to allow a customs border down the Irish Sea.

On Saturday DUP deputy leader Nigel Dodds had sounded positive about the Chequers deal.

He had said: "The Government's commitment at Chequers to the political and constitutional integrity of the United Kingdom with no borders between Northern Ireland and the rest of the United Kingdom is a welcome reaffirmation of what is an absolute priority for us."

However, Mr Wilson's amendment appears to go further, with a demand for legislation to cement the position.

Mr Rees-Mogg said: "The amendments will put into law the Government's often stated position that Northern Ireland should be treated the same way as the rest of the country.

"They will also ensure reciprocity of customs collection, and treating the UK and EU as equals.

"They will put into law the Government's stated position that we will not be part of the EU VAT regime.

"They will finally require any customs union should be created by primary not secondary legislation, so removing a Henry VIII power."

Speaking to The Sun newspaper, Mr Rees-Mogg added: "Unfortunately Chequers was a breakdown in trust. Brexit meant Brexit, but now it appears Brexit means remaining subject to European laws. I believe this will help the Government stick to the promises it made.

"It may resolve the dilemma the Prime Minister faces. Does she rely on Labour votes to achieve Brexit or does she change her mind and go back to Lancaster House. Will she stick to her earlier words?"

But Liam Fox, the International Trade Secretary, earlier told a Commons committee that the Chequers plan was designed to be a "credible offer" to the EU in order to take the negotiations forward.

"We hope that they will respond positively to that," he said.

Sinn Fein MEP Martina Anderson said: "This faux outrage from the DUP and their fellow travellers shows their lack of awareness of what is actually happening on the ground already and displays just how out of touch and far removed from the Brexit negotiations they actually are.

"This move shows the DUP is more interested in cleaving itself closer to the hard-right of the Tory party than acting in the best interests of people in the north.

"Checking facilities of the type they are now trying to block have existed along the Irish Sea for decades and have protected the integrity of the agriculture industry and animal welfare standards on the island of Ireland."

Belfast Telegraph