Ashurst Morris Crisp telecoms partner Stewart White is leaving to become director of public policy at mobile phone giant Vodafone from the end of March.
The highly-rated White has been handed the brief of co-ordinating Vodafone's worldwide dealings with regulators.
Vodafone's regulatory work is set to expand significantly after it successfully beat off rival bidder Bell Atlantic last month to agree a £37.5bn merger with US mobile phone company Air Touch.
The combined group, to be called Vodafone Air Touch, will be the world's largest mobile phone company.
White, who joined Ashursts five years ago from Denton Hall, said: "Vodafone approached me. After I thought about it, I thought I would regret it if I did not take it. It is an extremely exciting company."
He stressed that he will not be working as a lawyer for Vodafone – but he will be liaising with company secretary and head of legal, Stephen Scott, and his team.
He added that Ashursts, which has a 17-strong telecoms team headed by Chris Ashworth, had been "very positive" about his move. A spokeswoman for Ashursts said that the firm "wishes Stewart well and looks forward to having an on-going working relationship with him".