Heath Lambert has won several key accounts from Aon's Newcastle office within months of taking a team of top executives from the regional office.
Sources close to Heath Lambert said the Newcastle office had successfully poached three accounts worth in excess of £1m brokerage.
It is understood that manufacturing and distribution giant Saint Gobain, national baker Greggs, and construction mammoth Barratt Homes have all moved to Heath Lambert.
Top Aon brokers, including Michael Anderson, David Forster, Andrew Allan, David Bridges and Christopher Sutton, have recently defected to Heath Lambert.
The spat continued as Aon threatened to sue its rival unless it received compensation for the losses incurred by the moves.
The broker said it would sue in the High Court for breach of contract and inducement to breach if Heath Lambert did not admit liability and pay up.
Heath Lambert denied any wrongdoing. A source close to the broker said: "Aon was just trying to flex its muscles, and it doesn't seem to have a leg to stand on."
The lawsuit is yet to be issued to Heath Lambert.
A spokesman for Aon declined to comment on customer relationships. Heath Lambert also declined to comment.