BBC Three's monthly user reach has dropped more than 60% in the five years since the service switched to an online-only proposition, according to new research from Enders Analysis published in Broadcast.
In 2015, the last full year that BBC Three operated as a TV channel, the monthly audience stood at 22.7 million but that number had fallen to just 8.8 million by 2019.
The figure includes linear viewing of BBC Three content on BBC One and BBC Two, which accounted for the lion's share of BBC Three audiences, suggesting that linear TV is still a major driver.
The research does not account for the significant upsurge in BBC Three viewing during this year's coronavirus lockdown, led mainly by the runaway success of Normal People.
"In the context of youth channels like BBC3, this raises doubts about whether young audiences are, in the words of former director general Tony Hall, 'more ready to move to an online world'," said Neil Thurman, author of the report.
"Viewers are freer to choose what to watch now, unrestricted by the linear schedule, and my study indicates they might be using this freedom to watch a different mixture of programmes than before."
Earlier this year the BBC suggested that it may look at reviving BBC Three as a linear TV channel in the near future, potentially at the expense of BBC Four moving to online or being offered as a subscription service.