Nabarro Nathanson has been hit by the departure of its top energy partner, Mark Saunders, to US firm Dewey Ballantine.
Saunders, who headed Nabarros' energy group, is joining the New York-based international law firm in the next few months.
He says: "Deweys is expanding in England and has a policy to recruit and grow its overseas offices."
He says that the parting from Nabarros' is amicable, but that he had received "a competitive offer" from Deweys.
The firm specialises in project finance and cross-border litigation.
A spokesman for Nabarros plays down the significance of the departure.
"After 15 years with one firm it is natural to look around, and we would wish to continue working with him at Deweys," he says.
The two remaining lawyers, Robert Tudway, joint head of the energy group, and Gareth Jones, a specialist in oil and gas, will continue to run the energy practice until a replacement for Saunders is found.
Nabarros is also recruiting into its project finance group and is expected to announce new appointments in the next couple of weeks.
John Byrne, a company and commercial partner who left Nabarros to go to a US firm Dorsey & Whitney earlier in the year, says: "US law firms are able to offer a lucrative package, which is challenging and without any of the bureaucracy which is found in some large city firms."