In London, where the onslaught on BBC radio services by new advertising-funded stations such as Kiss FM and Melody Radio has been greatest, the figure is nearly 60 per cent.
Yesterday, the first quarterly figures for the radio-listening audience were released since the launch of Richard Branson's Virgin 1215 station, showing that in the station's first seven weeks, more than 3.2 million listeners a week tuned in, giving it a 2.2 per cent share of total radio listening time. Combined with continued success for the two other young national commercial stations, Classic FM - nearly 4.5 million listeners a week - and Atlantic 252, which has 3.7 million a week, commercial radio is eating into the loyalty of even the most traditional BBC radio audience.
The Radio Advertising Bureau (RAB), a body run by the commercial radio industry, estimates that 41 per cent of listeners to Radio 4's Today programme also tune in to commercial services, as do 42 per cent of those who tune in to The Archers.
'Commercial radio is now taking a stranglehold over the BBC,' Douglas McArthur, managing director of RAB, said. 'This is true in London, where commercial radio has doubled its share of the listening audience in six years to nearly 60 per cent, and it will be repeated throughout the country as the Radio Authority licenses new stations in other conurbations.'
Virgin 1215 is reaching 7.2 per cent of the total population, against the 8 per cent the station predicted. This compares to a 34 per cent reach for Radio 1, which is drawing 15,695,000 people a week, a fifth of all radio listeners.
'We're spot on target,' David Campbell, Virgin's chief executive, said. 'Radio 1 broadcasts on FM and AM, it has been running for 26 years and has a budget paid for by licence payers of pounds 37m - there is no comparison with Virgin.'
While he denied reports that the station would offer rebates to some advertisers, he admitted that their audience figure was being kept down because of the problems of broadcasting on the AM frequency: 'If we had FM we'd be doing better.'
The BBC fiercely denied that it was losing out to commercial services. It emphasised that two-thirds of the adult population listens to a BBC radio station each week, more than 31 million people. As well as improved performances for BBC local and regional services, Radio 5 measured a handsome increase in its audience of half a million to just under 4.2 million listeners over a week.
'When 24 of our 39 local stations and 3 of our national regional stations have increased their share of listening, it does not feel like a stranglehold,' said Ron Neil, managing director of regional broadcasting.
He said that improving audiences for local and regional services was proof that John Birt's decision to introduce greater speech programming did not deter listeners.
'We were told more speech would lose audiences, but it has nudged them up. Speech does not make people turn off, bad broadcasting makes people turn off.'
But, however well it has fared in the face of its first national rock competition from Virgin, the BBC's most popular station, Radio 1, has lost 400,000 listeners and a percentage share in the past quarter. In the past nine months, Radio 1's share of the listening audience has fallen from 22.4 per cent to 19.9 per cent, a drop to alarm Mathew Bannister, the incoming controller. The BBC's second biggest station, Radio 2, is down nearly 700,000 listeners, but maintaining its share.
'Virgin's inroads are small,' said Sue Stoessl, head of broadcasting research at the BBC. 'Radio 2 could be down because of the weather.'