BBC director-general Tony Hall has warned he cannot rule out more cuts as he confirmed that digital channel BBC3 is moving online to save £50 million a year .
The channel will be replaced with a BBC1 +1 service and an extra hour of children's programmes and some of its existing shows will be shown on BBC1 and BBC2.
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: "T his 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 BBC1.
BBC director of television Danny Cohen, a former controller of BBC3, 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 BBC3 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 BBC3 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 Tuesday.