Thomas, Carroll chief executive Stuart Love has resigned in what is seen as a "major blow" for the Wales-based broker.
It is believed Love stood down at the end of June after a disagreement with company chairman John Moore.
One broker source said: "John had gone into retirement but decided that he wanted to come back and work on until he was 65. In the meantime Stuart had empowered the team and got everybody working. He was a great people person.
"But when John Moore came back he changed a lot of things that Stuart wasn't happy about. Things came to a head about a month ago, and Stuart issued John with an ultimatum. John decided to stay and Stuart left."
The source said: "John Moore will live to regret this decision."
A senior insurance figure added: "We see this as a major blow for the broker. We will wait to see where Love re-emerges and continue to watch how Thomas, Carroll performs without him."
Love, a popular member of the broking community, left Thomas, Carroll at the end of June to go on holiday and failed to return.
He is thought to have received at least six jobs offers and speculation mounts as to where he might emerge. "He is on gardening leave until the end of the summer," the broker said.
Another added: "We think Stuart Reid [managing director of Stuart Alexander] was the first one in with a job offer."
Thomas, Carroll has appointed Rhys Thomas, a member of the Thomas family and board member, as Love's successor.
Thomas, Carroll said it would put out a statement early next week.