Half a million young BBC3 viewers could be lost to the Corporation when the channel goes – how can it get them back?

But now the stark reality has dawned. The channel is effectively ceasing operations in January and will close down for good in March. In the first months of next year it will essentially be a portal promoting BBC3 as an online broadcaster.

The Trust’s own public-value assessment said the Corporation’s proposal offered “low value for money” because of the reduction in viewers, but recognised that the savings would “offset financial pressures” or be used to re-invest in other areas.

But perhaps the most shocking statistic of yesterday’s announcement was the research figure that shows exactly how many viewers the BBC could lose.

There is a sizeable constituency of young viewers who only consume BBC television through BBC3 and have enjoyed a range of programmes including Professor Green’s examination of male suicide, the Bafta-winning Our War series which looked at the experience of soldiers in Afghanistan and hit dramas and comedies like In the Flesh and Bluestone 42. These are people who simply do not watch BBC1, BBC2 or BBC4.

Of these 925,000 viewers who consume no BBC TV content except BBC3, 80% of them – 740,000 people – could be lost to the BBC altogether, according to the Trust's projected figures.

And of that 740,000, 540,000 are people aged between 16 and 34. That is more than half a million young people who could theoretically be gone when BBC3 effectively dies as a channel in January.

Of course Auntie wants to get them back, and this week it outlined how it is hoping that these people will watch the BBC3 content that will air on BBC1 and BBC2 and that they will also “tune in” (if that’s still the right phrase) to BBC3 online.

The Trust says the general aim is for BBC1 and BBC2 to air two hours of “long form” BBC3 programming each week, "across the schedule". It is hoped that this will attract more young people to these channels, though the Trust defended its decision not to impose a quota of hours from the outset.

"It is not our job to schedule BBC content," Trustee Richard Ayre told RadioTimes.com, insisting that the BBC executive needs to have the "freedom" to master its own schedule.

But how likely is this to be successful? Of course some young people do watch BBC1 and BBC2. EastEnders, Strictly and Sherlock have a sizeable core of viewers aged 16 to 34. But there still seems a real danger that the rest could leave the BBC in droves and never come back.

When RadioTimes.com put this to the BBC Trust its representatives admitted it was unable to estimate how many viewers will turn off BBC3 altogether by failing to discover its content online or on BBC1 or BBC2. But it said that it obviously expects some of the more than half a million young viewers to sample the new online BBC3 in some way.

“What was expressed [in the 80% figure] was a possible loss of reach and it depends how fast and extensively BBC3 online is taken up and how successfully the BBC3 programmes and wider commitments to younger audiences on the two flagship channels succeeds,” said BBC Trustee Suzanna Taverne.

Fellow Trustee Richard Ayre added: “That 80 per cent figure is… a possible projection of what would happen if we closed BBC3. We have put in place all the mitigations… that are aimed at reducing that figure substantially... Of course as audiences take up the online offer that figure will reduce of its own accord.”

But will the new-look BBC3 appeal to young people? And will they even know what it is they are consuming?

“BBC3 is not closing, we are reinventing online,” says its controller Damian Kavanagh. ”We will not be a scheduled 7pm to 4am linear broadcast TV channel but we will be everywhere else giving you the freedom to choose what to watch when you want."

He is adamant that commissions in the vein of Asian Provocateur, Josh and Murder In Succesville will continue – “content that makes [viewers] think, makes them laugh and gives them a voice,” as he puts it – but £6 million per year, 20% of the overall budget, will be set aside to create new 'short-form' content: parodies, viral videos, show spin-offs, even things like listicles and photo stories.

"Split between our editorial pillars this will include short form video, picture led stories, animation, authored pieces, basically any way we can tell a story most effectively for our audience,"said Kavanagh. "We will no longer be limited to traditional TV."

“We will launch new content strands centred on topics that matter to young people like relationships, online life, crime and health. These are in development but we will issue a detailed brief to independent production companies very soon detailing what we are looking for and how to pitch ideas.

"But most importantly we will put young people at the heart of new BBC3 making them part of decision making, giving them a voice and a say in what we do.”

This is clearly going to require a marketing strategy to get content in front of prospective viewers in the first place but without a channel to go to it seems likely that many people who even sample the content online will lose their sense that they are connecting with BBC3 – and the BBC – at all.

It is an often repeated canard that in today's multiplatform digital world, allegiance to TV channels is dwindling. People sample content all the time and viewers have an increasingly diminished sense of channel loyalty.

As one BBC insider put it me today: “People will just think there is a funny Frankie Boyle thing they are watching online and they won’t think of it as BBC3. BBC3 isn't really a channel any more. That is a big problem. The BBC needs to recruit a new generation if it is to survive for the coming decades and this seems a strange way to do that."

There is certainly truth in that. Today, over 50% of video viewed by 16-24 year olds is not live TV and over 90% of that demographic own a smartphone and have at least one social media account. In 2003 it was 0%.

But channels aren't dying, whatever the BBC says about embracing young people's tendency to watch online. E4 and ITV2 posted a growth in audience earlier this year and without a place on the EPG BBC3 is in many ways a channel with a name but not a home.

I talked to many people within the BBC and to independent producers following the announcement and many of them say the same thing. They are too timid to say it publicly but few people in the content creation business – writers, producers, commissioners – are happy with the proposals.

"The BBC is going to wipe out a £1bn investment – the money it cost to create the BBC brand over the last 14 years – and for what? An online channel?," says one TV producer. "Anyone can set up an online channel. In America, where people know about the business of TV better than anywhere, channels are booming. Investors are ploughing money into TV channels because that is the best place for excellent content. This is the age of TV and the broadcaster. And for the BBC pull back from a channel providing programming for young people, diverse audiences and new talent – well it's a disaster."

We'll find out whether that's true in January.