Boris Johnson has been rebuffed by Brussels after demanding major changes in a new Brexit deal.
The Prime Minister set out his call for the backstop – the contingency plan to avoid a hard border with Ireland – to be scrapped from the divorce deal ahead of the October 31 Brexit deadline.
But European Council president Donald Tusk defended the measure and warned that those seeking to replace it would risk a return to a hard border on the island of Ireland.
Mr Tusk said: “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 reestablishing a border. Even if they do not admit it.”
Mr Johnson had written to Mr Tusk on Monday night outlining his opposition to what he called the “anti-democratic” Northern Ireland backstop.
In the letter, Mr Johnson said while he wants 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, the Prime Minister 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 did the same.
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, as part of the future relationship between the UK and EU.
Mr Johnson also leaves the door open for a fallback option – which the backstop was intended to be – in case other arrangements are not in place, saying “we are ready to look constructively and flexibly at what commitments might help”.
But the European Commission rejected the proposals and backed Mr Tusk’s assessment of the letter.
Commission spokeswoman Natasha Bertaud said “the letter does not provide a legal operational solution to prevent the return of a hard border on the island of Ireland”.
“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.”
Ireland’s premier Leo Varadkar had already rejected Mr Johnson’s call to reopen the Withdrawal Agreement during a telephone conversation with the Prime Minister on Monday.
Mr Johnson will meet German Chancellor Angela Merkel on Wednesday and France’s Emmanuel Macron on Thursday for his first face-to-face talks with Europe’s key powerbrokers.
The latest row with Brussels comes as a senior US politician warned that legislators would move to block a future US-UK trade deal if it puts the Good Friday Agreement at risk by introducing a hard border.
US Senate Democratic leader Chuck Schumer has written to US secretary of state Mike Pompeo warning that Congress could work on a cross-party basis to block a deal.
Mr Schumer called for the Trump administration to stop “over-promising an unconditional and unrealistic” post-Brexit trade agreement with the UK.
Mr Johnson spoke to US President Donald Trump on the phone on Monday night to update him on Brexit ahead of their meeting at the G7 summit in France at the weekend.
Mr Trump later said he and Mr Johnson 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 bloc.