Chris Evans, who took over Terry Wogan's breakfast show this morning, stands to lose as many as one million listeners, BBC executives fear.
Wogan (71) presided over the Radio 2 show, which attracted 8m listeners, for 27 years.
BBC bosses fear that Evans (43) will alienate many 'Togs' -- Terry's Old Geezers or Gals.
A BBC source said: "There is an expectation that the audience will drop. All new shows drop listeners at first. We will give it time to bed in and everyone accepts that there will be change, not just in demographic but some who are very fixed in their ways will toddle off somewhere else."
Evans had about 5m listeners on his Radio 2 Drivetime show.
The effect of the handover will not be known until April, when the quarterly Rajar radio listening figures are published.
Evans has set himself a target of 12 months to win back the lost listeners.
"We should be back to where Terry is pretty soon, if not further. [Our audience] might go down, but it will go up again within a year. That is what usually happens," he said.
Asked whether Togs would tune in, he said: "Some will and some won't. Hopefully more will than won't."
Kevin Joslin, who runs the Togs website, said that he expected die-hard Wogan fans to switch off. He said: "There are people that do not like his style."
Evans promised that there would be no repeat of the bad behaviour that got him sacked from Radio 1 and Virgin Radio.
He said that his new show would appeal to all ages.
"Let's forget the names [Togs] and let's do a show for everybody. If you are seven or 107 or anywhere in between or either side.
"We want to do a family show. There will be nothing to offend, lots to be hopeful about."
Evans has brought back the newsreader Moira Stuart (60) to woo older listeners.