Britain is to give £50m towards helping to save the second-largest rainforest in the world, the Congo Basin in central Africa.
In one of the Budget's most eye-catching and unusual items, Mr Brown announced an £800m Environmental Transformation Fund, to help developing countries cope with environmental changes such as global warming - and the Congo forest will be the recipient of its first major grant.
The money will form the basis of a new Congo Basin rainforest conservation fund, to be set up under the aegis of the 10 African countries surrounding the great wilderness, which at 700,000 square miles in extent is twice the size of France, but is increasingly threatened with development in the way the Amazon has been affected in Brazil.
Britain has persuaded Professor Wangari Maathai, the Kenyan environmental campaigner and 2004 Nobel Peace Price winner, and Canada's former Prime Minister, Paul Martin, a long-standing advocate for debt relief and for African leadership in development, to oversee the fund's establishment and advise on its governance and financial management, ensuring that it has strong African ownership and supports the needs of the Congo Basin countries.
"Fifty million local people rely on the tropical rainforest of the Congo Basin for food, shelter and their livelihoods, while the world relies on it, and other rainforests, as an ecological handbrake on our rapidly changing climate," said the International Development Secretary, Hilary Benn.
"But deforestation is a serious problem with nearly 6,000 square miles - that's equivalent to the area of North Yorkshire, Lancashire, Derbyshire and Cheshire combined - being destroyed every year.
"The UK's initial contribution of £50m to this African initiative - which we hope other countries will support -will help empower local people to live and work in the forest, while helping to prevent the double tragedy for them and the world that deforestation would bring."
The aim is to ensure that local people's livelihoods and rights are protected while helping them to better manage the forest and find livelihoods consistent with forest conservation.
The new fund will strengthen the work of the donors who are already active in the region, including Belgium, Canada, France, Germany and the US, and it will open a channel for new donors to add their support.
The Stern report on climate change, published in October last year, highlighted the fact that deforestation is responsible for 18 per cent of world greenhouse gas emissions, and said prompt action to tackle deforestation was a critical part of the global response to climate change.