The UK has cut £150m from its contribution to a global project which combats AIDS, tuberculosis and malaria.
The pledge of £850m for the period of 2026 to 2028 represents a 15% drop from the £1bn contributed for 2023-25.
The UK has historically been one of the biggest donors to the Geneva-based Global Fund, which raises and invests money to fight AIDS, tuberculosis and malaria.
It comes as European nations face growing pressure from US President Donald Trump to commit more funding to defence.
Foreign Secretary Yvette Cooper said the latest funding was an "investment in our shared security and prosperity" and it would help to save "up to 1.3 million lives and protect British people and our NHS from the spread of diseases".
But some charities and aid groups have criticised the drop in funding, calling it "disappointing".
Joanna Rea, director of advocacy for UNICEF in the UK, said: "Cutting the UK's pledge to the Global Fund to fight AIDS, tuberculosis and malaria jeopardises vital partnerships that protect children from preventable diseases.
"It's a disappointing decision and comes as the UK prepares to co-host a replenishment conference and should be encouraging other donors."
Earlier this year, the government cut its overall aid budget from 0.5% to 0.3% of gross national income so it could divert funds for annual defence spending.
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The UK is also set to co-host a pledging event for the fund with South Africa later this month in Johannesburg as world leaders gather for the G20 summit.
The Global Fund is trying to raise $18bn (£13.7bn) for its work in the coming three-year period.
The money will help it save 23 million lives and move the world closer to ending the three deadly infectious diseases, it says.
In October, Germany committed €1bn (£882m), which was €300m (£265m) lower than its pledge in the last funding cycle.
(c) Sky News 2025: UK cuts funding to AIDS, tuberculosis and malaria project by £150m
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