DONALD Tusk has rejected out of hand Boris Johnson’s demand to scrap the Irish backstop, suggesting the Prime Minister has no realistic alternative and, consequently, is supporting “re-establishing” a hard border on the island of Ireland.

The brush-off from Brussels came after Mr Johnson set out his objections to what he described as the “anti-democratic” backstop in a four-page letter to the outgoing European Council President.

But Mr Tusk took to social media to defend the measure and warned that those seeking to replace it ran the risk of a return to a hard Irish border.

He tweeted: "The backstop is an insurance to avoid a hard border on the island of Ireland unless and until an alternative is found.

"Those against the backstop and not proposing realistic alternatives in fact support re-establishing a border. Even if they do not admit it," declared the Council President.

Mr Tusk’s firm response will do little to ease UK-EU tensions as Mr Johnson prepares to travel to Berlin to meet Chancellor Angela Merkel tomorrow and to Paris on Thursday to meet President Emmanuel Macron.

The PM is also expected to meet Mr Tusk at the G7 meeting of world leaders in Biarritz at the weekend.

On Monday night, Mr Johnson wrote to the Council President outlining his opposition to the backstop.

In his letter, the PM said that while he wanted the UK to leave the EU with a deal, he could not support any withdrawal agreement that "locks the UK, potentially indefinitely, into an international treaty which will bind us into a customs union and which applies large areas of single market legislation in Northern Ireland".

As an alternative to the backstop, Mr Johnson said the UK would agree to a "legally binding commitment" not to put in place infrastructure, checks or controls at the border with Ireland and would hope the EU would do the same.

He argued the backstop should be replaced with a commitment to put in place "alternative arrangements" as far as possible before the end of the transition period, set at December 2020, and as part of the future relationship between the UK and EU.

The PM also left the door open to a fall-back option - which the backstop was intended to be - in case other arrangements were not in place, adding: “We are ready to look constructively and flexibly at what commitments might help."

But the European Commission also rejected the proposals and backed Mr Tusk's assessment.

Natasha Bertaud, its spokeswoman, said Mr Johnson’s letter did “not provide a legal operational solution to prevent the return of a hard border on the island of Ireland".

She explained: "It does not set out what any alternative arrangements could be and, in fact, it recognises that there is no guarantee that such arrangements will be in place by the end of the transitional period."

Meanwhile, a briefing note sent to EU diplomats accused the PM of spreading "misleading" and "incorrect" information about the backstop.

Leo Varadkar, Ireland’s premier, rejected Mr Johnson's call to reopen the Withdrawal Agreement during a near hourlong telephone conversation with the PM on Monday.

In response to Mr Tusk’s tweet, Simon Coveney, Mr Varadkar’s deputy, said: “This has been the clear and consistent EU position; it remains so.”

Downing St again made clear that the UK Government would not re-establish a hard border on the island of Ireland.

“We are deeply invested in the peace, prosperity and security of Northern Ireland and always will be and we have been clear that we will never place infrastructure, checks, or controls at the border,” said a spokesman.

“But it is clear that unless the Withdrawal Agreement is reopened and the backstop abolished there is no prospect of a deal. It has already been rejected three times by MPs and is simply unviable as a solution, as the PM’s letter makes clear.

“We are ready to negotiate, in good faith, an alternative to the backstop, with provisions to ensure that the Irish border issues are dealt with where they should always have been: in the negotiations on the future agreement between the UK and the EU,” he added.

The latest row with Brussels comes as a senior US politician warned that Congress would move to block any future US-UK trade deal that jeopardised the Good Friday Agreement by introducing a hard border.

Chuck Schumer, the US Senate’s Democratic leader, has written to Mike Pompeo, the US Secretary of State, warning American lawmakers could work on a cross-party basis to block such a deal.

He called for the Trump administration to stop "over-promising an unconditional and unrealistic" post-Brexit trade agreement with the UK.

On Monday night, Mr Johnson again spoke to Donald Trump on the phone to update him on Brexit ahead of their meeting at this weekend’s G7 summit.

The US President later said he and the PM had shared a "great discussion" in which they talked about moving "rapidly on a US-UK free trade deal" after Britain's departure from the Brussels bloc.