CHI ended its three-year tenure of the Heart creative assignment earlier this week when Global opted to consolidate the accounts of all three stations into a single agency.
The win is expected to result in a £4m billings boost for The Law Firm, which was set up by the St Luke's founder Andy Law in 2005. Law was unavailable for comment.
The agency is believed to have been given the account after a speculative presentation to Global Radio executives during the summer. Media planning and buying arrangements through Starcom and MediaVest are unaffected.
The Law Firm already has experience handling media accounts, having taken over creative work on the £22 m Daily Mail business in August last year.
It lost the £10m creative account for the Daily Mail's stablemate the Evening Standard to McCann Erickson three months ago.
Separately, Martin Smith, one of The Law Firm's partners, has left the agency. It is not known if the former Grey chief executive and Bartle Bogle Hegarty deputy chairman has a job to go to.