A witness said the Prime Minister made the remark after trying to push for a quick agreement on the future rights of British citizens in the EU, and vice-versa, at the Brussels summit.
It led to an awkward impasse, because the rest of the EU has demanded the right to exclude Britain when they discuss their side of the Brexit negotiations.
They have also insisted there will be “no negotiations without notification” – meaning they must await the triggering of the Article 50 exit clause, at the start of next year.
“It was a difficult moment for diplomatic etiquette,” one leader’s aide told The Times newspaper.
“‘I think I’d better leave now,’
she said. She was very polite, but it was a bit embarrassing,” he added.