DIBB Lupton Broomhead's major client Du Pont has defected to Eversheds in a deal worth hundreds of thousands of pounds to the firm.
The Swiss chemical giant, which had paid several UK law firms to advise on environmental, employment and corporate law, held a beauty parade to choose a new principal legal advisor in March.
Dibbs, Eversheds and an unnamed London firm were in a final shortlist of three.
It is believed a Dibbs lawyer even travelled to Geneva to try to persuade the Swiss chemical giant to keep its work with the firm - but to no avail.
Paul Smith, Evershed's head of environmental law, led the pitch for the firm from its Leeds office with partner Peter Norbury. Norbury said the firm's appointment would bring in "hundreds of thousands of pounds" of annual fee income.
Eversheds was chosen, he said, "because we have a good spread of offices around the country. Du Pont's have plant up and down the UK and they needed to know that there would be someone able to visit any one of their sites reasonably quickly."
Robin Smith, Dibbs' national senior partner, said: "Where we are involved in this sort of situation it is our policy not to discuss it."