Pinsent Curtis Biddle is losing one of its highest billing partners to US firm Arnold & Porter.
Jeremy Willcocks is understood to be among some of the highest billers at Pinsent Curtis despite being a salaried partner. At Arnold & Porter, however, he will take equity as well as become head of corporate.
One source close to Pinsent Curtis says Willcocks' decision to leave is partly because the firm did not give him equity, despite his significant contribution to profits.
The source adds that Willcocks "fell through the net" regarding promotion when he agreed to move to the commercial department to help build the London team.
It is understood that Pinsent Curtis makes up only a few salaried partners to equity each year and this has led to a logjam within the salaried ranks.
Willcocks refuses to discuss partnership. He says of his move: "It was a different opportunity. I want to build on what Arnold & Porter has already got, to open doors with US clients and to develop my own clients.
"There is no great plan to take on X number of people but I plan to develop a full service practice."
Willcocks joined Pinsent & Co - which merged with Simpson Curtis in 1995 - seven years ago from Slaughter and May.
Although he is a corporate lawyer, he also worked in the projects department, acting on major deals such as the 1997 Connex rail privatisation and the Luton Airport project.
However, all three resignations were made before the merger was announced.
In a statement, a Pinsent Curtis Biddle spokesman says: "Jeremy was an unsuccessful candidate in our equity partner promotion rounds last year.
"While the firm has a good track record of working with disappointed candidates it is inevitable some who do not succeed will pursue their careers elsewhere."