Christopher Hall, head of the London office of US firm O'Melveny & Myers, has left to become executive vice-president and general counsel of Dutch telecoms company KPNQwest.
O'Melveny learned of Hall's decision early this month and Hall took up his new post on 17 January.
He will be responsible for KPNQwest's legal, financial and regulatory affairs and will be part of the company's management team.
Hall took over as partner-in-charge of the Los Angeles firm's London office in 1986. He is credited with refocusing its business and turning round a stale practice.
London partner Christopher Kandel says: "Christopher has built up the office from two lawyers to 10. He gave it its focus on finance, reorganisations and workouts which are other kinds of finance."
O'Melveny worked on KPNQwest's $1bn-plus flotation last year, and Kandel hopes Hall's move will cement the firm's relationship with the company.
"This is quite exciting for the office here because KPNQwest has been an interesting client, but not a huge one, and I would suspect that it will become an important client for us.
"It's a very exciting opportunity for Christopher because his job will be general counsel for a rapidly expanding telecoms company and he's also going to have a big role in the development side in planning strategic acquisitions."
The firm will bring a partner to London from the US and will decide on who will head the office in the next few weeks.
O'Melvenys' managing partner, C Douglas Kranwinkle, says: "Christopher has made significant contributions to our firm.
"We are delighted by this opportunity to strengthen our relationship with KPNQwest."