The breakdown in talks came after Prime Minister Boris Johnson and Chancellor Angela Merkel of Germany had a phone conversation in which, according to one British official, Ms. Merkel dashed hopes of a deal. But the details of the call, which Britain called “frank” and Germany called “ friendly ,” are murky.

The E.U. called Mr. Johnson’s approach a “stupid blame game” and warned him against the histrionics.

Details of the call: One British official, speaking anonymously, faulted Ms. Merkel for the failed talks, saying she had told the prime minister that disagreements over Northern Ireland could not be bridged, “not just now but ever.”

But Germany said Ms. Merkel did not go beyond the E.U.’s previously disclosed objections to the border proposal .

Strategic moves: The prime minister’s political survival in a likely election depends in part on looking like he is being forced, kicking and screaming, into this reversal. In an election that might happen as early as next month, he faces a threat from the hard-line Brexit Party — which will seize on any perceived weakness in his stance.