Britain is to double its contribution to the Global Fund to fight Aids, tuberculosis and malaria.
Millions of Africans have HIV
The Department for International Development will give £100 million to the fund in each of the next two years.
The increase means Britain will account for 20% of the total global financial support for the battle against Aids.
In total the UK will spend £1.5bn on fighting HIV and Aids over the next three years - a large part of which will be spent in Africa.
This includes the UK's contribution to the Global Fund and DFID's bilateral programmes, which support prevention, testing, counselling and the care of orphans.
Figures show that 25m adults are infected with HIV in sub-Saharan Africa alone.
International Development Secretary Hilary Benn said: "The UK is committed to the fight against Aids.
"The Global Fund needs more money, and we hope other donors will also significantly increase their contributions."
The Global Fund, established in 2002, is funding projects in 127 countries.
As a result of the fund's activities more than one million people have received voluntary HIV testing, 130,000 patients have been given anti-retroviral treatment, and 350,000 people have been trained to fight Aids, TB and malaria.
Mr Benn also announced that DFID would provide an extra £8 million to UNAIDS, the United Nations programme on HIV and Aids.
This cash will help support a Global Task Team set up with the UN, World Bank and Global Fund, following a summit in London in March, to ensure that increasing Aids resources reach those who need support and services.
He said: "This will allow UNAIDS to advise countries on how to use their Aids funding more effectively."