The prime minister has ruled out merging the Department for International Development and the Foreign Office.

The plan had been mooted as a potential means of creating a "super ministry" for Gordon Brown to take over after the general election.

But pressed by Liberal Democrat leader Charles Kennedy in the Commons on Wednesday, Tony Blair insisted the idea is "not under consideration" in Whitehall.

Acknowledging "some nervousness" in the Foreign Office and other areas of government when it was created by Labour in 1997 he paid tribute to the Department as "probably the most respected [aid ministry] in the world".

"I would want to keep it that way," he added during a session of prime minister's questions.

Kennedy had earlier quizzed Blair on Britain's presidency of the G8 group of the world's richest nations this year.

The Lib Dem chief welcomed proposals outlined by Brown for a "new Marshall Plan" for Africa which would raise international aid for the continent by half a trillion dollars in the next 10 years.

But he said the idea could "only be realised if the United States can come on board" adding that a way to persuade President Bush of the case for change would be to tie it up with plans for "good governance" in African states.

Blair replied that Brown's proposed international finance facility was "one way, not the only way, we can extend the aid available".

And he "entirely agreed" with the case for good governance, saying the issue is central to the Commission for Africa's work.