DAC Beachcroft will launch on 1 November after members of both Beachcroft and specialist construction law firm Davies Arnold Cooper approved the proposed merger.
The new law firm will employ more than 2,000 staff and will have a turnover in excess of £175 million with offices all over the world.
Beachcroft managing partner Paul Murray said: “We are very pleased to be creating a firm with such considerable strength across its core markets.
“Both firms specialise in the insurance sector and the combined firm will provide insurance clients with a distinctive, full-service offering in the UK and internationally.”
Davies Arnold Cooper senior partner Danny Gowan said: “The merger brings together complementary strengths in both our teams and the business processes that underpin how we work with clients.
“In addition to the benefits it provides to our insurance clients, it doubles the size of each firm’s existing real estate practice.
He added: “I am particularly encouraged that both firms have similar cultures which will help ensure a successful merger.”
The new firm will have around 230 partners and will have eight UK locations including London, Manchester and Birmingham.