The BBC is due to outline its plans to close five of its 32 World Service language services, according to BBC.
Staff are due to be informed of the redundancy details on Wednesday, and it is thought that about 650 jobs will be lost from a workforce of some 2,400.
The Macedonian, Albanian and Serbian services will be axed, as will English for the Caribbean and Portuguese for Africa, in a bid to save £46m a year.
Director general Mark Thompson said it would be "a painful day" for the BBC.
Writing in the Telegraph, he said the cuts would "inevitably have a significant impact on the audiences who use and rely upon the relevant services".
Yet he said they were "consistent with our long-range international goals and strategy" and that "supporters of the international role of the BBC should not despair".
Unions have called the moves "ferocious" and have condemned the "drastic cuts".
Speaking on Radio 4`s Today programme, Jeremy Dear, general secretary of the National Union of Journalists (NUJ), said that the World Service was "vital" and "should be protected".
The service, which started broadcasting in 1932, currently costs £272m a year and has an audience of 241 million worldwide across radio, television and online.
Last October the government announced the BBC would take over the cost of the World Service from the Foreign Office from 2014.
According to Mr Thompson, the cuts were necessary due to last autumn`s Comprehensive Spending Review.
The BBC will make a statement on Wednesday about the latest wave of redundancies, to be phased over two years.
It is understood that two-thirds of the jobs will go in the first 12 months.
A reduction of programmes in another seven languages is also set to be announced.
The NUJ said it would hold a demonstration outside the World Service headquarters in central London on Wednesday.
It has also 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 the plans.
The NUJ said the "drastic cuts" would "severely damage the national interest of the UK".
"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," general secretary Jeremy Dear said.
BBC global news director Peter Horrocks said the closures were "not a reflection on the performance of individual services or programmes".
"They are all extremely important to their audiences and to the BBC," he said.
"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.
"We need to focus our efforts in the languages where there is the greatest need and where we have the strongest impact."
Former World Service managing director Sir John Tusa described the cuts as "bad, bad, bad".
Speaking on the Today programme, he said: "I think it`s awful for World Service listeners because they won`t have access to the programmes, and it is awful for British foreign policy because they are weakening substantially one of the most important elements of international cultural diplomacy."
On Monday, the BBC said it would cut about 200 websites and up to 360 posts from its online division as part of a plan to reduce its budget by £34m.
Among the websites set to close are teen services Switch and Blast, and the 606 football forum.
The corporation said the changes were intended to make its website more distinctive, and to reduce competition with commercial sites.