LONDON — Succumbing to what appears to have been a disastrous urge to brag about his super-important connections, a BBC correspondent unexpectedly declared in a radio interview on Tuesday that Queen Elizabeth II had once told him she was “pretty upset” about the presence of a radical Islamist cleric in North London.

It is considered a shocking breach of etiquette to reveal what, if anything, the queen tells you, especially when it comes to political views, which she is technically not supposed to have. The BBC immediately issued an abject apology, saying that the correspondent, Frank Gardner, was completely out of line.

“The conversation should have remained private, and the BBC and Frank deeply regret this breach of confidence,” the broadcaster said in a statement. “It was wholly inappropriate. Frank is extremely sorry for the embarrassment caused and has apologized to the palace.”

Buckingham Palace said it would not comment on the matter.

The episode occurred on BBC Today, an influential public-affairs morning radio program. Mr. Gardner, the broadcaster’s security correspondent, was speaking about Abu Hamza, an Egyptian cleric who settled in Britain and preached violent anti-British jihad at a North London mosque.