Wragge & Co has lost its position as the Rover Group's main legal adviser to local rival Eversheds.
Eversheds' Birmingham office has taken Wragges' place, after the national firm represented the Phoenix consortium that bought the Rover Group from BMW for a nominal fee of £10.
The consortium had to fight off a bid from venture capitalist Alchemy Partners for the car manufacturer.
The change coincides with an overhaul of both the Rover Group's and BMW's legal departments.
The overhaul will affect the department under the control of Andrew Armitage, previously company secretary and legal director at the Rover Group and now head of legal for BMW Service, a subsidiary of the German car giant. Armitage intends to step down at the end of the year.
The internally promoted Jane Ruston, a former Wragges lawyer, now heads the Rover Group's two-strong legal department as company secretary and legal director.
Ruston says Eversheds has not acted for the Rover Group before but adds that Wragges, which has advised the company for 10 years, will not lose out completely because it will represent the company when conflict issues arise.
She says: "Because of the acquisition of the business and because Eversheds was involved with that, they are familiar with the company going forward so it is a continuity thing."
In his present role, Armitage heads the businesses retained by BMW, which include the manufacturing plant at Cowley. Ruston is in charge of the Rover unit which held onto the Longbridge plant.
Land Rover was sold to Ford in May for £1.8bn. Armitage says: "The business has effectively been broken down into three units and I am staying on to manage the transition."
He declined to say if he has secured another job but says that he intends to remain in-house. Armitage has been with the company since 1984 and before joining worked in local government.
Regarding his departure he says: "It was an easy decision to make."
Eversheds and Wragges were unavailable for comment.