The US legal market is preparing for one of the biggest mergers in its history with the news that Dewey Ballantine and Orrick Herrington & Sutcliffe are in merger talks.

New York-based Dewey and San Francisco's Orrick confirmed this week that they were in talks regarding a merger, which would mark the most significant tie-up in the market since the 2004 union of Wilmer Cutler & Pickering with Boston's Hale and Dorr. A combination of the top 50 outfits would create a firm with almost 1,500 lawyers and revenues of just under $1bn (£535m).

The merger talks are the latest in a series for Orrick. Last year, the firm absorbed a chunk of Paris firm Ram-baud Martel and also took on the London and Moscow offices of Coudert Brothers. However, talks with Californian rival Cooley Godward collapsed in 2003.

Orrick has been growing rapidly and last year achieved a 14% increase in turnover to $554m (£296m). Dewey had a less successful year with turnover increasing 3% to $392m (£210m). Both firms recorded average profits per equity partner of $1.2m (£640,000).

Dewey chairman Morton Pierce said: "We have been looking to expand aggressively in Europe and this would help accelerate that expansion."

One ex-Orrick partner commented: "It looks good on paper, but the firms have very different cultures."