Disaffected employees of the renowned radio service believe that its editorial independence will be abolished by his plans for the corporate restructuring, abruptly announced in June.
They argue that the quality of its news service will be reduced by the move to absorb its newsroom into the BBC news division dominated by the domestic agenda.
They are also angered by his decision to stop the World Service making its own English language arts, business, drama, music, sport, science and religion programmes. These will be bought from the BBC's production division.
More than 1,350 staff have signed a petition to "save the World Service" and last month saw a lobby of Parliament on the issue, which the former head of BBC World Service, John Tusa, has branded "the greatest act of cultural vandalism" he has ever seen.
But Mr Birt told a press conference that the restructuring of the organisation would go ahead as planned. "We are going to have to continue to explain the considerable benefits which will come," he said.
Questioned about whether the reorganisation would lead to job losses, he refused to commit himself.
"People talk about the World Service like it's a statue in the garden which needs preserving," he added. "But where we have applied those ideas [of separating commissioning from production] they have been conspicuously and demonstrably successful."
It was a question of economics, the director-general continued. The World Service, which has 140 million listeners, would face a pounds 10m gap within the next couple of years between income and expenditure.
"There is a major competitive change to the markets in which it broadcasts and it needs a more flexible, loose-limbed structure for the digital age to allow it to adapt its services and make them ever more creative - and their costs ever less.
"We will take that argument to the doubters and to our own staff and in the end we will win it, because they are good and strong arguments."
The fightback by the BBC's most senior staff continued last night when Sir Christopher Bland, the corporation's new chairman, defended the reorganisation in a speech to the Radio Academy in Birmingham.
The World Service would retain its editorial independence and would continue to remain "a separately managed directorate within the BBC", he told delegates to the conference.
"The authority of the World Service will not be reduced by the changes. John Birt and I have given an undertaking to the Foreign Office, and I and the Board of Governors would not agree to proposals which risked any diminution in quality.
"The World Service's programmes will remain distinct from those in other areas of the BBC. There is no question of it being swallowed up by some homogenising BBC new machine."
Sir Christopher added that it was "the one service which was mentioned to me time and time again by the 400 or so people who wrote to congratulate me when I became chairman.
"The message was the same: Look after the World Service."It was not a question of "if it ain't broke, don't fix it," he continued. "I say: the World Service may not be broke now, but we want to ensure that it doesn't become broke in the future."Reuse content