Hilchos Lashon Hara, chapter 2, section 11

Seven leaders of a town met to discuss something, and decided by majority vote. One leader was against the decision. He may not tell people that he thought the opposite, but the others disagreed with him and decided against him. That is lashon hara.

This is forbidden not only when they agreed to keep the discussion secret. It is forbidden in any case – even to hint at it in a way that lets the listener understand that he disagreed with the other leaders.

It also makes no difference whether he says it on his own, or whether someone attacks him with curses and insults about the decision. Either way, it is forbidden to tell.