Chicago-based Mayer, Brown & Platt is the latest US firm to bolster derivatives in its seven-partner London office with two senior appointments.
The move will strengthen the international firm's role in work between London and the US, the two most important markets for derivatives.
US lawyer Richard Cole, partner in charge of the London office, said: "This gives us a very big boost, although we have done work in derivatives here before. With this acquisition, I think we will be clearly the strongest Anglo-US derivatives practice in London."
Mayer Brown, with over 250 partners worldwide, 25 in derivatives, has recruited specialist lawyer Eric Bettelheim as head of its derivatives team in London. Bettelheim comes from the London office of US competitor Rogers & Wells, where he was a partner.
Mayer Brown's other recruit, Denis Petkovic, joins as partner from Stephenson Harwood, which he has worked for in London and Hong Kong.
Petkovic was formerly a lead partner in Stephenson Harwood's derivative products group. His strength is in emerging markets derivatives, particularly structured eurobonds, notes and warrants.
Cole said Petkovic's broad transactions background would bring him into other areas of work in the firm apart from pure derivatives.
Petkovic said: "The nature of my derivatives practice, involving creation of bonds, notes and other structured products, increasingly needs heavyweight and seamless service on the US side as the products are sought to be marketed in the US."
The move to recruit and expand is client-driven, with firms facing increasing demands to have capabilities on both sides of the Atlantic, especially in capital markets, fund management, commodities and financial regulation, said Cole.
However, flexing its muscle in London may not bring the firm into direct competition with UK capital markets giants.
Cole said: "We will be pitching for work with international and very often American clients...[but] it will be less attractive for us to pitch for primarily local work."
Another US firm pushing into London's capital markets sector is Weil Gotshal & Manges, currently driving to recruit UK lawyers to open for business on 1 March.
However, capital markets lawyer Nick Eastwell, a partner involved in Linklaters' derivatives group, said: "There is a lot of this going on. One is never complacent, but one is always sceptical about the ability of these outfits to break into the field.
"Unless you have critical mass, getting plenty of back-up from partners and assistants, you can't hope to survive."
But Petkovic responded: "Global expertise and quality of service is what counts, not necessarily size."