Costas Andrea, a senior Penningtons lawyer, has left the firm to set up his own niche practice, passing up a partnership offer in the process.
He takes another assistant, a secretary and "300 cases" with him to form the firm Andrea & Co, which is based in Old Woking in Surrey.
Andrea had been with Penningtons for nine years, building up his own specialist self-contained holiday insurance practice within the insurance department. He revealed that he had signed an non-restrictive covenant agreement with the firm allowing him to take his clients and his team, providing he did not try to poach any further clients from the personal injury department.
He said: "I operate a very specialist practice, and have a network of agents across the world, so I took my opportunity to go it alone and carve out a specialist niche for myself, concentrating exclusively on holiday insurance."
A spokeswoman for Penningtons described the agreement as "positive and amicable".
She added: "We have made strong progress in identifying key practice areas and personal injury remains a priority.
"We retain a substantial personal injury team within the larger litigation practice."
Penningtons restructured last year. It made redundancies and a number of partners resigned voluntarily.