LONDON — After surviving the latest round of infighting in her divided cabinet, Prime Minister Theresa May of Britain was in a good enough mood to speak to reporters after she boarded her plane to the Group of 7 summit meeting in Canada, late Thursday.

But upon landing she was met with leaked remarks from her wayward foreign secretary, Boris Johnson, and, as so often, Britain’s most senior diplomat had hardly been diplomatic.

Mrs. May’s government, said Mr. Johnson, lacks “guts” in its negotiation over British withdrawal from the European Union, or Brexit, which the foreign secretary champions. He dismissed the importance of the Irish border, a major sticking point in negotiations with the European Union, saying it was inconsequential.

“It’s so small and there are so few firms that actually use that border regularly, it’s just beyond belief that we’re allowing the tail to wag the dog in this way. We’re allowing the whole of our agenda to be dictated by this folly,” he said in comments leaked to BuzzFeed.