GCap has been served a dossier by Ofcom concerning an investigation into whether its "Secret Sound" competition breached the regulator's broadcasting code.
The radio firm has been given up to two weeks to reply to the contents of the media watchdog's dossier.
A senior source close to GCap said: "GCap was told in June 2007 that Ofcom would not be pursuing an investigation into this and now suddenly it has decided to, just as GCap is in the middle of takeover negotiations with Global Radio."
The source added: "The burden of proof is now on Ofcom to show who suffered from GCap's actions."
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