BBC Radio 1 has said it will look "long and hard" at its output after its weekly audience figures fell below 10 million during the last three months.
Sara Cox will not be leaving as Radio 1 breakfast host
But the station said there was no radical overhaul around the corner despite losing almost 500,000 listeners between April and June.
There was good news for BBC Radio 2 in the Radio Joint Audience Research Limited (Rajar), which showed it attracted a "record" share of the listening public over the three-month period.
The BBC also claimed success because a record number of people are listening to the BBC's World Service in the UK.
And in the commercial sector, London's 95.8 Capital FM defied investor worries by increasing its share of the market.
NUMBER OF LISTENERS EACH WEEK
All radio: 43.6 million
All BBC: 31.9 million
All commercial: 31.4 million
BBC Radio 1: 9.8 million
BBC Radio 2: 13 million
BBC Radio 3: 2 million
BBC Radio 4: 9.6 million
BBC Radio Five Live: 5.8 million
Classic FM: 6.5 million
Capital: 2.6 million
Virgin Radio: 2.7 million
talkSPORT: 2.1 million
The figures count listeners over the age of 15 who tuned in for at least five minutes.
Radio 1 is now listened to by fewer than 9.9 million people per week - the lowest since a new method of counting was introduced in 1999.
Under a previous counting method, it attracted 16.5 million 10 years ago.
"It's not been a good quarter for us - we make no bones about that," a spokesman said.
"We need to look long and hard at the network as a whole to check what needs to be done to arrest any further decline in reach and share."
It was too early to know why the figures had dropped, he said.
But BBC Radio 1 - with its commitment to new music - was having to compete with commercial stations playing the biggest hits non-stop.
Any remedy would involve more than just replacing DJs and breakfast host Sara Cox - often considered to be in the firing line - would remain in her job, the spokesman added.
If all listeners over the age of four are taken into account, BBC Radio 1 has lost 800,000 of them over the last three months.
There is speculation about Jonathan Ross' future at Radio 2
Sister station BBC Radio 2 - which overtook BBC Radio 1 as the nation's favourite in 2001 - reached 16.3% of the audience over the last three months.
That was the highest since the new counting method was introduced - although 200,000 fewer people listened in the last three months.
The rise in share and drop in audience was possible because the total number of people listening to all radio stations went down by more than one million.
There have been reports that current bbc Radio 2 host Jonathan Ross has been approached to join Capital after their contract with star presenter, Chris Tarrant, expires in November.
Capital's audience share rose by 9%, the figures showed.
Listening figures to its breakfast show reportedly increased - even though Tarrant, one of its star DJs, was on holiday.
At the BBC World Service, an increase of 68,000 listeners means almost 1.5 million people now listen in the UK - boosted by its presence on digital TV, digital radio and the internet.
BBC Radio Five Live was down by 612,000 listeners, BBC Radio 4 lost 340,000 listeners and BBC Radio 3 lost almost 100,000 listeners, the figures showed.
The furore over Andrew Gilligan "boosted Radio 4's figures"
Jenny Abramsky, the BBC's director of Radio and Music, said: "This is an impressive share performance by the BBC which still commands 53% despite the fact that more stations than ever are being measured.
"Realistically we must expect that this share will gradually be eroded as even more stations come aboard."
Meanwhile, earlier unofficial figures suggested that the row over the government's Iraq dossier helped BBC Radio 4's Today programme dramatically improve its audience, according to figures.
Under a radical new system of measuring listeners, developed by GfK media research and backed by Kelvin McKenzie, Today broke the four million listeners barrier for the four weeks following the storm.