US-based international law firm Mayer Brown and Hong Kong’s Johnson, Stokes and Master are merging in a move that will create the world’s tenth-largest law firm by revenue.

The rare trans-pacific merger reflects the increasingly global operations of large businesses. It brings together two firms founded in the 19th century which are both largely focused on commercial transactions and litigation.

The combined entity, which will be known in Asia as Mayer Brown JSM, will have an annual revenue of $1.3bn and around 1800 lawyers. Jim Holzhauer, Mayer Brown chairman, will chair the global firm’s central policy and planning committee, and Elaine Lo, chairwoman of JSM’s partnership board, will head the combined entity’s Asia board.

The firms expect to grow substantially after the merger. Mr Holzhauer projects annual revenue to hit $2bn ”very quickly” and Ms Lo predicts earnings of $4bn within two to three years. ”This kind of growth cannot be obtained by just organic growth alone,” said Ms Lo.

Up to now, Mayer Brown’s Asia presence has been limited to one office in Hong Kong and a representative office in Beijing. JSM, whose clients include HSBC, Bank of China and Cathay Pacific, has 200 lawyers in mainland China, but none outside Asia.

That has meant that both firms had to find outside partners when handling cases that involve elements outside the firms’ home regions, such as cross-border mergers and acquisitions.

”The synergies will come from the clients,” said Paul Maher, Mayer Brown vice-chairman. ”Many of the major banks or industries we represent have transactions that will have both a European and Asian component, for example, and we will soon be able to do both within one firm.”

The merger comes nearly six years after the trans-atlantic tie-up between the US’s Mayer, Brown & Platt and the UK firm of Rowe & Maw to create Mayer Brown. The firm has suffered a tumultuous year, most recently with partner Joseph Collins being indicted in Manhattan on fraud charges related to the collapse of trading firm Refco in 2005. Mr Collins has pleaded not guilty to the charges.

Ms Lo said the merger brings together JSM’s knowledge of the Chinese market with Mayer Brown’s global reach and experience in sophisticated commercial transactions. ”The world is becoming more globalised,” she said. ”Chinese companies are encouraged by the central government to expand overseas and they are just poised to grow out of China.”