EU chief Donald Tusk has hit back furiously at Boris Johnson's Brexit demands in a Twitter slapdown.
The European Council President suggested Britain's PM was "not proposing realistic alternatives" after he demanded a key part of the Brexit deal was ripped up.
And he accused Mr Johnson of making a hard border in Northern Ireland more likely due to his refusal to compromise.
While Mr Tusk did not name the UK Prime Minister in his tweet, the target of his slapdown was clear.
His message came barely 12 hours after Mr Johnson wrote a formal letter to Mr Tusk demanding the "Irish backstop" clause is removed from the Brexit deal.
Mr Tusk 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 reestablishing a border.
"Even if they do not admit it."
European Commission spokesman Daniel Ferrie added: "The letter doesn’t provide a legal, operational solution to prevent the return of a hard border on island of Ireland.
"It does not set out what any alternative arrangements could be.
"In fact, it recognises that there is no guarantee that such arrangements will be in place by the end of the transition period.
"As stated on many occasions, we stand ready to work constructively within our mandate."
The backstop is a key part of Britain's 585-page Withdrawal Agreement with the EU, agreed by Theresa May last November.
It would prevent border checks springing up between Northern Ireland and the Republic if there is no arrangement in time. But in return it could trap the UK in EU customs rules indefinitely.
Mr Johnson's Brexit wish list, sent to Brussels last night, said the backstop is "anti-democratic and inconsistent with the sovereignty of the UK" and could weaken the Northern Ireland peace process.
He said it should be removed, to be replaced with "alternative arrangements" by the end of 2020 while Brexit happens in the meantime.
Yet he did not make clear exactly what "alternative arrangements" it should be replaced with.
In Irish premier Leo Varadkar said the Withdrawal Agreement could not be reopened.
And Labour claimed Mr Johnson's four-page letter was a "fantasyland wishlist".
The blunt response deepens the deadlock for Boris Johnson a month after he took power - and just 72 days before the October 31 Brexit deadline.
He is due to meet Germany's Angela Merkel tomorrow, France's Emannuel Macron on Thursday and Irish PM Leo Varadkar next month. Mr Johnson is also expected to meet Mr Tusk this weekend at the G7 summit.
