Britain hasn't offered a "realistic alternative" to the Irish backstop, European Council President Donald Tusk said in a tweet responding to Prime Minister Johnson's letter requesting that Brussels drop the policy.
"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."
The European Commission said it agreed with Tusk's position, as was stated by their Deputy Chief spokesperson Natasha Bertaud.
"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 recognizes that there is no guarantee that such arrangement will be in place by the end of the transitional period. Otherwise and of course, we do stand ready to work constructively with the UK and within our mandate."
Johnson says the UK won't reopen talks unless the backstop is dropped, calling it anti-democratic and incompatible with British sovereignty.
But he's under mounting pressure to avoid a no-deal withdrawal from Europe, after leaked reports warned of food, fuel and medicine shortages and mayhem at the border if Britain crashes out.
Johnson said on Monday he was confident Brussels would be open to negotiations but both sides appear to be digging in, even as the clock keeps ticking.
In a marked toughing of its position, the UK says that, instead of a gradual phase out, EU rules of free movement will end immediately if there is a no-deal Brexit on October 31.
For now, EU officials say they're waiting for the UK parliament to reconvene in September, when the opposition is expected to try to oust Boris Johnson's government.