A&O Shearman, as the firm will be known, will have about 3,900 lawyers and and some 800 partners across 49 offices, including Belfast and Dublin. The two firms have 250 years of combined experience.

Wim Dejonghe, senior partner at Allen & Overy, said: “This combination of two great firms is such an exciting step for us. Both firms have a history of excellence, and together we think A&O Shearman will be a firm unlike any other in the world.

“We have listened to our clients and their requests for the highest quality advice to help navigate the demands they face, and to do so in an integrated and globally consistent way. We, A&O Shearman, will do this by accelerating our ability to bring the best of both firms, regardless of geography.”

Adam Hakki, senior partner at Shearman & Sterling, said: “Client need for global elite firms has never been greater. They are calling for integrated global legal solutions and advice: merging with Allen & Overy will dramatically accelerate our ability to meet their needs in an increasingly complex environment.

“Allen & Overy is an outstanding firm whose work we have long admired and thought of as a kindred spirit. We have both always placed great emphasis on attracting and retaining top talent, were early to globalize, and are relentlessly focused on quality, excellence, and collaboration.”

The proposed merger is subject to customary closing conditions, including a vote of the partners of each of the respective firms. Lazard is serving as financial advisor and Simpson Thacher & Bartlett LLP is serving as legal counsel to Allen & Overy. Davis Polk & Wardwell LLP is serving as legal counsel to Shearman & Sterling.