Donald Tusk accused Mr Johnson of supporting the return of a hard border in Ireland by being against the policy.
The President of the European Council tweeted his terse response after the PM wrote a four-page missive telling Brussels time was now “very short” to get a withdrawal agreement passed before 31 October.
But Mr Johnson said MPs would not vote for anything which still contained the backstop, which he branded “anti-democratic and inconsistent with the sovereignty of the UK”.
And he said despite them “showing a little bit of reluctance at the moment” he was “confident” Britain’s “friends” in the EU will agree to change the existing Brexit deal.
However Mr Tusk rebuffed him, saying: “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.”
His comments were echoed by the European Commission, who said the PM does “not provide a legal operational solution to prevent the return of a hard border” in Ireland/
Spokeswoman Natasha Bertaud said: “We welcome the UK Government’s engagement and continued commitment to an orderly withdrawal.
“We firmly believe this is in the best interests of both the EU and the UK.
“However, we also note that 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.”