The BBC World Service is to close five of its language services with hundreds of job losses expected.
It is thought about 650 jobs will be cut from a workforce of some 2,400 in a bid to save £46 million a year.
Union leaders said the "ferocious cuts" were a "direct result of the Government slashing funding to an internationally respected and successful broadcaster".
The corporation said in a statement: "BBC World Service has announced that it will be closing five language services - Albanian, Macedonian, Portuguese for Africa and Serbian; as well as the English for the Caribbean regional service.
"This is part of its response to a cut to its Grant-in-Aid funding from the UK's Foreign and Commonwealth Office.
"The cut is part of a BBC World Service restructure in order to meet a 16% savings target announced in the Government's spending review of October 20th last year."
BBC Global News Director Peter Horrocks said: "These closures are not a reflection on the performance of individual services or programmes. They are all extremely important to their audiences and to the BBC.
"It is simply that there is a need to make savings due to the scale of the cuts to the BBC World Service's Grant-in-Aid funding from the UK's Foreign and Commonwealth Office and we need to focus our efforts in the languages where there is the greatest need and where we have the strongest impact."
The BBC said it would not confirm details of job losses before briefing staff later today.
Horrocks is due to hold a press conference at the World Service headquarters in central London where he will give "significant details" of the service's response to funding cuts.
But a report on the BBC website was predicting 650 job losses with redundancies phased over two years and two-thirds of the jobs going in the first 12 months.
It also predicted the announcement of a reduction of programmes in another seven languages.
The World Service, which started broadcasting in 1932, costs £272 million a year and has an audience of 241 million worldwide across radio, television and online.
The National Union of Journalists will stage a demonstration outside the World Service head office later today.
The union has written to the chairman of the House of Commons Foreign Affairs committee Richard Ottaway and the chairman of the Culture, Media and Sport Committee John Whittingdale, calling on them to review thoroughly the BBC's plans for cuts in the service.
NUJ general secretary Jeremy Dear said: "These ferocious cuts to a valued national service are ultimately the responsibility of the coalition Government, whose policies are destroying quality public services in the UK."
He continued: "These cuts are a direct result of the Government slashing funding to an internationally respected and successful broadcaster.
"Journalists and other BBC employees are rightly angered at the destruction being caused to a broadcasting service of which the UK should be proud.
"The NUJ will join with other unions in defending jobs and quality broadcasting at the World Service.
"Our members have already explained, in a remarkably calm and reasoned way, why the cuts are such a severe threat to their service."
The corporation said yesterday it plans to cut its online budget by 25% from £137 million to £103 million by 2013/14, with the loss of up to 360 posts.
Director general Mark Thompson acknowledged the changes would be "painful" but insisted BBC Online "lies at the heart" of the corporation's digital future.