Partners at the UK’s Allen & Overy and US-based Shearman & Sterling have approved a plan to merge the two firms, creating a massive new competitor in the legal industry.

The deal was approved by both firms, with 99% of partners at each voting in favor of the combination, according to a joint announcement.

The new firm—A&O Shearman—joins the world’s largest, with roughly 3,900 lawyers and a combined gross revenue of $3.4 billion last year. It comes after both firms had explored tie-ups: A&O seeking a foothold in the US and Shearman looking for scale as its share of the legal services market declined.

“This is a historic moment for both firms and our profession,” Wim Dejonghe, A&O’s senior partner, said in a statement announcing the vote results. “We are delighted that our partners have voted so resoundingly in favor of this merger, which is a transformational step for the legal industry.”

Dejonghe and Khalid Garousha, A&O’s managing partner, will lead the newly combined firm, according to sources familiar with the situation. Adam Hakki, Shearman’s senior partner, is expected also to play a role in the new firm’s leadership.

A&O weighed a merger with US firm O’Melveny & Myers before talks fell through in 2019. Shearman & Sterling abandoned tie-up talks with Hogan Lovells in March.

A&O and Shearman painted a compelling public picture of strength of the combined firm, said Tony Williams, a consultant who previously held the top leadership position at Clifford Chance. That increased the stakes for getting partners on board.

“You’ve got to bear in mind the impact of not doing the deal,” Williams said in a June interview. “It will have meant both firms will have two sets of public merger discussions, neither of which they’ve done, so that would be cataclysmic in terms of the credibility of the leadership.”

Shearman has been reeling from the effects of partner exits and a slow deals market. The firm’s revenue declined by more than 10% last year, to $906.9 million. Manhattan litigator Adam Hakki took over the top role at the firm in April, shortly after the Hogan Lovells deal fell through, succeeding David Beveridge.

Some of London’s other Magic Circle law firms have also targeted stateside growth. Clifford Chance and Freshfields Bruckhaus & Deringer have largely focused on individual and group hires to expand in the US, after being raided by the top firms in their own backyards.