Millions of children are orphaned by Aids

International Development Secretary Baroness Amos said the fund would receive an additional $80m.

That will being the UK's contribution to the fund to two hundred and eighty million dollars over seven years, and mean it is committed to contributing until 2008.

The announcement follows a US pledge on Wednesday of up to $1 billion dollars for the fund.

Cash alone is not the only answer Nick Partridge, Terrence Higgins Trust

But charities are warning that the sums are still not enough to carry out the work needed.

They say it has a shortfall of around $1.4 billion.

'Humanitarian disasters'

Simon Wright, UK HIV campaign manager for Action Aid, told the BBC: "The headlines that have been announced are quite impressive. The story underneath is less impressive.

"It was envisaged that the fund should be able to increase the amount it's giving each round.

"And if its going to get to the kind of levels that the UN said were necessary, we should be seeing a round later this year of around $1.6 billion.

"The fund does not have that money."

He said action to tackle Aids, TB and malaria was essential.

"These are not just epidemics in their own right. They are humanitarian disasters and they are keeping these poor countries poor and driving people further into poverty."

International effort

Developing countries apply to the fund for grants to pay for prevention and treatment programmes to fight diseases which are the biggest killers in the developing world.

There have been concerns about the running of the fund. Last February, former International Development Secretary Clare Short told the Commons the Fund was "important but not being as well led as it might be".

Her successor, Baroness Amos, has said the government will look closely at how its money is spent.

She denied that the funding announcement meant a reduction in real terms.

She told the BBC: "We've said to the fund we are quite prepared to look again at the issue of additional funding.

"We give money, not just to the fund in regards to HIV Aids.

"We have a number of bilateral programmes with countries, we give money to the international Aids vaccine initiative, and we want to see the health systems in developing countries strengthened."

Political leadership

Nick Partridge, chief executive of Terrence Higgins Trust welcomed the extra funding.

But he added: "We know, from the way the HIV epidemic has gathered pace in richer countries in recent years, that cash alone is not the only answer.

"If this move by the government is underpinned by the political leadership and continued will to tackle HIV, both internationally and here in the UK, it will be doubly welcome."

He added the UK government must continue to contribute to the Global Fund, and work with other governments, UNAIDS and other organisations as well as encouraging drug companies to meet their obligations.

"We will only have a real impact on this devastating epidemic if we make full use of our combined expertise."