The British government said on Monday it would contribute 1 billion pounds ($1.18 billion) to the Global Fund to fight AIDS, tuberculosis and malaria, more than six weeks after other countries made their commitments.
The total, which covers 2023-25, is 30% less than Britain pledged during the previous funding round in 2019, and below the 1.8 billion pounds requested this time.
Historically, the country has been a major donor to the fund.
Its absence had generated surprise in global health circles when other leaders committed $14.25 billion on the sidelines of the United Nations General Assembly in September.
Other G7 countries increased their donations given the unprecedented need after the pandemic disrupted efforts to tackle other urgent health needs.
"This fund gives hope and opportunity to millions who would otherwise suffer," Andrew Mitchell, Britain's minister for development, said.
The Global Fund welcomed the funding, but advocacy organisations said it was not enough.
"For decades the UK has been a leader in the global response to these infectious diseases, but no more," Mike Podmore, director of STOPAIDS, said in a statement.
Malaria No More said the cut may mean some life-saving products that were ready to go, such as next-generation mosquito nets developed in Liverpool, Britain, would not make it to those in urgent need.
The Global Fund estimates that it has saved 50 million lives since its inception in 2002, but COVID-19 knocked progress off-course and the situation has not yet recovered, according to its annual report published in September.
Britain cut its overseas aid budget to 0.5% of national income in 2020 from 0.7% before, as the pandemic strained finances. It faces continued economic challenges as well as political turmoil. It has changed prime minister twice this year because of internal disagreement within the government.
($1 = 0.8502 pounds)
(This story has been refiled to fix typo in last paragraph to read 'It faces...')
(Reporting by Jennifer Rigby; editing by Barbara Lewis)
