Medium-sized US firm Duane Morris & Heckscher is opening an office in London by merging with three-partner City firm Geisler & Laws.
The move marks the first step outside the US for the expansionist Philadelphia practice, which now has offices in every financial centre in the US except Los Angeles and Atlanta.
The 400-lawyer firm is taking over Geislers, a six-lawyer niche practice which specialises in IT and motor industry work.
Geislers split away from City firm Rakisons in 1996. The firm is, in effect, part of the in-house counsel to big manufacturing clients, understood to include General Motors and Fiat.
Duane Morris is a full-service firm representing big US companies in corporate and commercial work, property and litigation. Its big manufacturing clients include Intel and Filofax.
Its profits were nearly £200,000 per partner last year, with gross revenues of about £1.5m. Geislers' profits are understood to be nearer £300,000 per partner.
Chairman of Duane Morris Sheldon Bonovitz says: "Now that we have a foothold in the US and are building out our offices, we have clients that have asked us to get into London. A lot of our clients are multinationals and this is an important marketplace."
Bonovitz says the firm is planning to move out of the vulnerable middle tier by doubling in size within three years. It is one of the fastest growing firms in the US having opened seven new offices there last year and taken on 100 lawyers.
Geislers partners Alexander Geisler, Susan Laws and Jonathan Passman all become partners of Duane Morris, with three other lawyers joining as assistants.
The firm will be looking to recruit in London, with plans to grow to more than 20 lawyers.