Angela Merkel had heard enough. After a ten-minute presentation by Theresa May and a discussion in which she had used the time-worn phrase “Brexit means Brexit”, the German Chancellor was running out of patience.

“What else do you want?” the exasperated Mrs Merkel demanded of Mrs May, after the Prime Minister had failed to explain what it was she actually expected her counterparts to offer.

For Mrs May, the inquisition was a new experience; at previous summits she has made her pitch to a silent audience, then left the room so they could all dissect her pleas behind her back.

And, according to sources who witnessed Thursday night’s exchange, it was a deeply uncomfortable moment for Mrs May.

“She just did not have an answer,” said one senior EU source.

Mrs May had made the strategic error of arriving for the two-day summit in Brussels without a written explanation of what she wanted the 27 other EU leaders to agree to.

Her language was admirable enough: “We must get this right,” she told them.

“Let’s hold nothing in reserve. Let’s get this deal over the line in the interests of all our people.”

But when the leaders probed her on exactly how the intractable issue of the Northern Ireland backstop could be solved, Mrs May appeared to be taken aback.