Coudert Brothers is understood to be in takeover talks with 26-lawyer Holborn-practice Barnett Alexander Chart.
The takeover will be the latest stage of Couderts' plans to expand to 100 English lawyers in London by 2001.
In September last year, the US firm took over specialist London property firm Debenham & Co – a move that surprised City-based US rivals who described it as “very, very domestic and parochial”.
At the time, Couderts' London managing partner Philip Burroughs said that the next target would be a “first tier” UK firm.
This year, Couderts has hired Lovell White Durrant banking partner Alexander Janes and litigation partner Bridget Wheeler from Holman Fenwick & Willan.
The firm has also taken up larger office space in the City.
Neither Coudert Brothers nor Barnett Alexander were available for comment.