BBC director-general Tony Hall has warned he cannot rule out more cuts as he confirmed that digital channel BBC Three is moving online to save £50 million a year.
The channel, which launched shows such as James Corden's Gavin And Stacey and Jack Whitehall's Bad Education, will be replaced with a BBC One +1 service and an extra hour of children's programmes and some of its existing shows will be shown on BBC One and BBC Two.
Tens of thousands of viewers have signed a petition opposing the move, which has been criticised by unions, but Lord Hall said it was "the right thing to do".
In an email to staff, he said the licence fee had been frozen while the demands on it increased.
He said: "This is the first time in the BBC's history that we are proposing to close a television channel. I can't rule out it being the last change to our programmes or services."
The plans, which are subject to approval by the BBC Trust, would save more than £50 million a year, with £30 million of that earmarked to go towards drama on BBC One.
BBC director of television Danny Cohen, a former controller of BBC Three, said the move was "the biggest strategic decision the BBC has made in over a decade".
He said that "in an ideal world we would not be making this move for a few more years".
"Given an entirely free hand, I would make this change in about four or five years' time, using the years between now and then to slowly shift the balance between linear and on-demand BBC Three content.
"That would be a safer, less risky strategy. But we don't have the choice to wait and do that due to the investments we need to make.
"I want to protect programme budgets from more major cuts across the board and the BBC has to find the money for new obligations, including the World Service, that will cost £350 million a year."
A spokeswoman for the trust said its "priority" would be to "listen to the views of audiences".
"Any major changes to existing BBC services require approval from the trust," she said.
"In this case, we expect to conduct a public value test, including a public consultation, so licence fee-payers will have the opportunity to have their say in the process."
Gerry Morrissey, leader of the technicians' union Bectu, expressed concern about the likely impact on jobs.
"We are extremely disappointed that the BBC has made this decision with very little consideration for its employees and freelance staff.
"We are being told they will not say anything about staffing before the end of the year, which is totally unacceptable.
"We will be consulting our members and will make representations to the BBC Trust."
Mr Morrissey said he did not believe that moving BBC Three from the screen to the internet would make it easier to connect with a younger audience, adding that the move was part of cuts caused by the freeze in the licence fee.
"Over 3,000 jobs have been lost, quality has declined and now content is suffering."
An online petition calling on the trust to "save the channel and to continue to invest and air new comedy, drama and music programming for young adults in its rightful home" has been signed by more than 65,000 people since it was launched on March 4.
Mr Hall told Radio 4's World At One that he had considered closing the more highbrow BBC Four but it would not have been as big a saving.
He said: "We did think about that and in fact BBC Two and BBC Four are working more closely together than they've ever done before, but I'll tell you why we didn't go there actually.
"One was the sum involved but, secondly, I go back to my belief that audiences are changing and the best way of serving our younger audiences, who really matter to us, was to serve them in that sort of way whereas the BBC Four audience is older."
Mr Cohen told Richard Bacon's 5Live radio show he could not guarantee the future of BBC4.
He said: "The honest answer to that is 'No, we can't say for certain what will happen to BBC4 in the future'.
"If future funding for the BBC comes under more threat, we will have to take more services along a similar route."