The Congo Basin, which spans 10 countries at the heart of Africa, loses 3.7 million acres a year to agriculture, logging, road development, oil exploitation and mining, environmentalists say.
The British aid was intended to protect the fragile region.
"The first stage is 50 million pounds sterling to sustain projects in 10 African countries to protect the tropical forests of the Congo Basin," Mark Bensberg, deputy British ambassador to Democratic Republic of Congo, told a news conference.
The money will be deposited in the African Development Bank, which will start an African environment fund as ordered by an African Union summit in January in Addis Ababa.
Congo Republic Finance Minister Henri Djombo said the fund would be strictly managed and would include representatives of foreign development partners on its board.
The 10 regional countries are seeking $2 billion (1 billion pounds) from foreign donors to fund a conservation plan agreed two years ago in Brazzaville and due to run until 2013. So far donors have raised just $300 million.
The Congo Basin comprises around 30 percent of the world's total vegetation, covering an area of 470 million acres (200 million hectares) and it is home to some 400 mammal species, including the world's largest populations of lowland gorillas, chimpanzees, bonobos and forest elephants.
It covers Democratic Republic of Congo, most of Congo Republic, the southeastern reaches of Cameroon, south Central African Republic, northern parts of Angola, Gabon, western Burundi and Rwanda, the islands of Sao Tome and Principe and mainland Equatorial Guinea.
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