The draft document is expected to say Northern Ireland might have to follow EU single market rules to avoid a "hard border", in lieu of other solutions.

Downing Street has dismissed any prospect of a return to a hard border.

But the DUP has said if the Irish Sea became a trade border it would withdraw its support for the UK government.

One of the party's senior members, Sir Jeffrey Donaldson, tweeted: "This fundamentally breaches the understanding reached in December and would undermine the constitutional status of NI in the Belfast Agreement.

"If the EU or Dublin believes the UK government will be signing up to a border in the Irish Sea, they are deluded. Taoiseach (the Irish prime minister) knows that."

Meanwhile, former Conservative prime minister John Major is due to give a speech on Brexit later.

He is expected to lay out his recommendations for a successful deal between the UK and the EU.

The EU commissioners' 120-page draft Brexit withdrawal document will refer to three possible options for avoiding physical infrastructure on the Irish border.

However, the only one to be fleshed out will be the government's least-favourite: the option of Northern Ireland staying aligned with European rules and regulations, says BBC Brussels reporter Adam Fleming.

The document will encapsulate - in legally binding text - agreements already reached on Ireland, citizens' rights and the UK's so-called "divorce bill".

It will form the basis of further negotiations with the UK in areas like the transition and might still be tweaked by the 27 remaining member states.

EU negotiator Michel Barnier has said the document will not contain any surprises because it translates the political pledges made by both sides in the talks so far.

According to reports by Irish broadcaster RTE, the text will say that Northern Ireland may be considered part of European Union customs territory after Brexit, alluding to a single "regulatory space" on the island of Ireland with no internal barriers.

Leaked letter

On Tuesday, a leaked letter from Boris Johnson has appeared to accept the possibility of future customs border checks on the island of Ireland after the UK leaves the EU.

In the letter, obtained by Sky News, the foreign secretary tells Theresa May 95% of traffic would still pass unchecked if there was a hard border.

Theresa May wants trade to be frictionless across the Irish border after Brexit.

But there is plenty of political friction as every potential solution seems to bring a new problem.

In a leaked letter designed to demonstrate that there would be no need for new infrastructure between Northern Ireland and the Republic, Boris Johnson used a potentially toxic phrase: "even IF there is a hard border".

This allowed critics to suggest that a regime of border checks which would be anathema to republicans, nationalists and the Irish government was being contemplated.

Downing Street swiftly reiterated its commitment to no hard border.

But one of the solutions - indeed, the most detailed option - being put forward by the EU would keep Northern Ireland aligned with EU regulations.

The DUP's Jeffrey Donaldson has said the government cannot sign up to what would in effect be a border in the Irish Sea.

And with rebellions threatened by some of her own backbenchers, Mrs May is likely to need the DUP's MPs to deliver the Brexit she's promising.

Earlier, Mr Johnson was criticised by opponents for suggesting in a BBC interview the issue of the border could be managed as easily as London's congestion charging zone.

In his letter to the prime minister, Mr Johnson seeks to play down the "exaggerated impression" of "how important checks are" at EU external borders.

Following the letter's emergence, Labour called for Mr Johnson - one of the leading Brexiteers in the cabinet - to be dismissed "before he can do any more damage".

A spokesman for Mr Johnson said the letter was "designed to outline how a highly facilitated border would work and help to make a successful Brexit".

"The letter points out there is a border now, and the task the (cabinet Brexit) committee face is stopping this becoming significantly harder," he said.

"It shows how we could manage a border without infrastructure or related checks and controls while protecting UK, Northern Ireland, Irish and EU interests."

More about: Brexit