Smart and Cook is set to acquire Northampton-based Hammon Osborne.
The broker said Hammon Osborne employs more than 40 staff and places £25m annually.
Hammon Osborne chairman Godfrey Hammon will continue to lead the business and will take a seat on the Smart and Cook board.
The financial consideration of the transaction was not disclosed.
Smart and Cook group chief executive Paul Meehan said: “Hammon Osborne is our biggest acquisition so far – it is a major independent broker with a business ethos very similar to our own.”