Ofcom also warned that the spin-off could put off commercial investment and new entrants into the market more generally, particularly among radio stations hoping to serve audiences left behind by the BBC.
The regulator said: “We consider that this creates a significant adverse impact on fair and effective competition which is not justified by the public value of the proposal.”
‘David vs Goliath victory’
The provisional verdict will come as a relief to commercial rivals who had warned of a “chilling” effect on the wider market.
Phil Riley, the co-founder and chief executive of Boom Radio, said he was “delighted” with the decision, which he described as a “David vs Goliath victory”.
He said: “Here’s a case where a new, innovative radio station like Boom Radio came on the market, did something new, got a great audience, and then was immediately attacked by the BBC.
“If that had been allowed to go ahead, every other commercial operator and every other independent would have said I’ve got a great idea but what’s the point in launching it because if it’s unsuccessful I’ll lose all my money and if it is successful the BBC will come along and clone it.”
Ofcom first intervened in the proposals last summer, ordering the BBC to carry out a public interest test.
The corporation admitted that the spin-off would have an impact on rivals but pushed ahead with its plans after tweaking its proposed schedule to include more speech content such as archive documentaries and interviews and adding regular news bulletins.
Mr Riley said he felt sorry for the producers and presenters at the BBC who would not be able to launch a new radio station, which he described as “the most fantastic thing”.
But he added: “I don’t feel sorry for the BBC because I don’t believe they should ever, ever have proposed this. I think they have been, I have to say, arrogant in how they’ve approached this whole process.”
Ofcom also said it intends to block proposals to extend the broadcasting hours of Radio 5 Sports Extra from a part-time service offering live sports to a full daily schedule of programmes.
The regulator warned the spin-off would have a detrimental impact on TalkSport, the station owned by Rupert Murdoch’s News UK.