Prime Minister Theresa May has effectively been told by the EU to choose between a Brexit deal and the DUP.

Amid growing frustration in Brussels, the EU's chief negotiator took the unprecedented decision to tweet what he is willing to offer the UK in a bid to break the ongoing impasse.

Michel Barnier has proposed that Britain can leave the customs union unilaterally, but Northern Ireland must remain tied to the EU regulations to avoid a hard border.

The idea was swiftly rejected by the DUP, which props up Mrs May's minority Government at Westminster, as "neither realistic nor sensible".

Its deputy leader Nigel Dodds said a deal was still possible but warned there must be less "intransigence" in Brussels.

It came as former Foreign Secretary and leading pro-Brexit MP Boris Johnson accused Mr Barnier of attempting to "annex" Northern Ireland. His accusation came in a message, posted in a private WhatsApp group, which has been seen by the Belfast Telegraph.

Referring to Mr Barnier's proposals, Mr Johnson posted: "Be careful of what Barnier is proposing now... seems to mean annexation of NI."

Mr Johnson did not respond to requests for comment.

The developments come ahead of a potentially decisive week.

Mrs May will put her withdrawal agreement to the House of Commons again on Tuesday in the hope of overturning the record 230-vote defeat she suffered in January.

However, she was left to desperately implore the EU to facilitate legally-binding changes to the controversial backstop yesterday.

The Prime Minister said she needed "one more push" to get the deal through Parliament.

However, EU sources said they remain "hugely sceptical" about her ability to win any vote.

Mr Barnier's offer includes a legal guarantee that Britain can leave the backstop at any time but it does not change the so-called 'Irish Protocol' within the withdrawal agreement.

He stressed Britain would still need to honour its commitment to preserve a border free of controls on this island.

"EU commits to give UK the option to exit the single customs territory unilaterally, while the other elements of the backstop must be maintained to avoid a hard border," he tweeted. "UK will not be forced into customs union against its will."

Sources said this would amount to a border "in the Irish Sea" if a future UK Government decided to break the customs link with the EU.

The backstop was originally designed to apply exclusively to Northern Ireland, but Mrs May convinced the EU to make it UK-wide after the DUP complained that the province was being treated differently.

A majority of business leaders and farmers in here believe a sea border would be less impactful than one on the island.

Mr Dodds categorically rejected the compromise proposal last night.

"This is neither a realistic nor sensible proposal from Michel Barnier," he said.

"It disrespects the constitutional and economic integrity of the United Kingdom. This is an attempt to get ahead of a possible blame game and appear positive when in reality it is going backwards to something rejected a year ago. As the Prime Minister has said, no United Kingdom Prime Minister could sign up to an arrangement which annexes Northern Ireland from the rest of the United Kingdom.

"Whilst the European Union has spoken often about their value of the peace process in Northern Ireland, this proposal demonstrates that they have a one-sided approach and a lack of understanding about the divisions in Northern Ireland.

"Just as nationalists and republicans oppose a new north-south border, unionists oppose any new east-west border which would place a new barrier between Northern Ireland and the rest of the United Kingdom.

"We need to work for a sensible deal which can work for everyone in Northern Ireland. It is possible but there must be less intransigence in Brussels." Brexit Secretary Stephen Barclay said now was not the time to rerun old arguments.

"The UK has put forward clear new proposals. We now need to agree a balanced solution that can work for both sides," he said.

The gulf which remains between the EU and UK was on display yesterday as both Mrs May and Taoiseach Leo Varadkar urged each other to change tact.

The Irish Government last night supported the proposal from Mr Barnier, saying he continues "to stand up for the interests of the whole of the EU, including Ireland".

"The Taoiseach has repeatedly made clear that he would not oppose a backstop which is specific to Northern Ireland, if that is deemed helpful, but that would have to be a decision for the UK," a spokesman said.

Speaking in Dublin, Mr Varadkar questioned why the UK hasn't come forward with a concrete offer to the EU.

He described Brexit as "a problem of their creation" and accused the House of Commons of actively going against the wishes of a majority of people in Northern Ireland who support the backstop.

He said both sides had spent 18 months agreeing the backstop.

"Bear in mind elements of that compromise, like extending the backstop on a UK-wide basis, the single customs territory involving all the United Kingdom, these were compromises that the British Government sought.

"We were and remain happy to apply the backstop to Northern Ireland only if they want to go back to that," he said.

With no changes to offer Parliament, Mrs May looks set to lose her second attempt to win approval of her deal.

Yesterday she issued a plea for support, saying "no one knows" what will happen if her plan is defeated, with the possibility of Brexit being derailed completely.

Mrs May said both the democratic and economic cases for backing her deal are "clear", and issued a plea to MPs: "Let's get it done."

The Prime Minister used her speech in Leave-supporting Grimsby to both urge the European Union to give ground on the backstop and call on Tory Eurosceptics to recognise that failure to back the deal could result in a softer Brexit or another referendum.

"Next week, MPs in Westminster face a crucial choice: "Whether to back the Brexit deal or to reject it," she said.

"Back it and the UK will leave the European Union. Reject it and no-one knows what will happen.

"We may not leave the EU for many months, we may leave without the protections that the deal provides. We may never leave at all," she said.

Rejection of the deal would be a "moment of crisis".

Foreign Secretary Jeremy Hunt warned Brussels that a failure to co-operate on securing changes to the Brexit deal could "poison" relations with the EU for years.

"This is a moment of change in our relationship between the UK and the EU and history will judge both sides very badly if we get this wrong," Mr Hunt told BBC Radio 4's Today programme.

Belfast Telegraph