The BBC Executive wants to move BBC3 - which has been home to shows from Gavin And Stacey and Cuckoo, to Sun, Sex and Suspicious Parents - online to cut costs.
In its provisional conclusions outlined today, the Trust said that it had some “clear concerns” about the short-term impact of the change, including “a potential impact on the ability of the BBC to try out new ideas and develop new talent”.
In the short term the online channel would be likely to lose viewers, having a “much smaller audience than the broadcast channel it is replacing”.
But it said that the plans should be approved, as the online service would save £30 million a year and be more distinctive than the current BBC3 channel, whose audience is falling.
It said that the move should be dependent only on the Executive agreeing to several conditions, including clearer commitments to shows on BBC1 and/or BBC2 which appeal to younger audiences.
The announcement is bad news for campaigners who have battled to keep BBC3 alive as a TV channel and have called the move “disastrous” for the fostering of “new talent” and “innovative ideas”.
Harry Potter star Daniel Radcliffe, Broadchurch actress Olivia Colman and Poldark’s Aidan Turner are among those who have signed an open letter on the issue.
More than 290,000 people have signed a petition to “save BBC3”.
Jimmy Mulville, who runs production company Hat-Trick, behind hit shows including Room 101, Father Ted and Have I Got News For You, has said the proposed move online would be “the kiss of death” for the channel and leave it “competing with huge behemoths like Netflix and Amazon who are spending hundreds of millions of dollars on one show”.
The Trust said that it recognised that there could be a “risk that (the online channel) may have less appeal to producers, writers and actors, impacting the shows that are commissioned.”
After today’s judgment, a further 28-day consultation period will take place with the Trust before a final decision is made later this year, and the channel could close as early as January.
The Trust said that the BBC would need to agree to conditions to mitigate against the short term loss of younger audiences.
But in a blow to the BBC Executive, it also said that plans for a BBC1 plus one channel should be rejected.
BBC director-general Tony Hall announced the plans for the flagship channel in 2013.
But the BBC’s governing body said that a plus one channel would be at the expense of commercial rivals and reduce the profitability of ITV and Channel 5.
It has said that plans for the BBC iPlayer, to allow viewers to watch more shows before they are screened on TV, should be approved.
The Executive originally said that plans to move BBC3 online, as a service targeted at 16 to 34-year-olds, would save £50 million a year, but the Trust said that it had taken other costs into account to bring the figure down to £30 million.
BBC Trust chairwoman Rona Fairhead said: “It is clear that the long-term future of broadcasting is online and the BBC needs to find new and exciting ways to help audiences make that transition, while bearing down on costs overall.
“We know young audiences are already moving towards the online future, but we do recognise that in the short term some of them will feel the immediate impact of the BBC Three proposals.
“We are therefore asking the BBC for commitments to ensure it uses the full range of its television services to better serve young people and others who make up BBC3’s audience.”
The BBC wants to use savings from the closure of the BBC3 channel to invest in drama on BBC1.
BBC3 supporters have been hoping to follow in the footsteps of campaigners who successfully saved BBC 6 Music from closure in 2010.