The funds are designed to help vulnerable countries adapt to climate change

The funds were set up four years ago and a number of other European nations have already donated millions.

The UK has pledged money but not paid up, the New Economics Foundation claims, despite climate change being a key issue for its presidency of the G8.

A government spokesman said the UK would make its first payment in June.

Global warming is an ecological debt that the richest countries owe to the poorest countries Andrew Simms, New Economics Foundation

"The UK is the largest international donor - providing �10m - to the Special Climate Change Fund and we are making our first payment next month," a Department for International Development (DFiD) spokesman told the BBC News website.

Uncertain future

The other fund, which deals specifically with Least Developed Countries - two-thirds of which are in Africa - has not been promised any UK money as yet.

According to Andrew Simms, of the New Economics Foundation, other countries including Germany, Sweden, Canada and Denmark have already poured millions of euros into the two funds.

He says the absence of British contributions is an embarrassment, particularly since the UK is so vocal on the issues of climate change and Africa.

"We see global warming as an ecological debt that the richest countries owe to the poorest countries," he told the BBC. "So it is tremendously ironic that as the poorest countries continue to pay debts that they can't afford, rich countries can't pay for the ecological debt of climate change."

The UK government says that it is committed to the fund, but needs to investigate its viability first.

"We need to be sure that the Least Developed Countries Fund helps developing countries cope with climate change and represents a good use of taxpayers' money," the DFiD spokesman said.

"We must agree ways of ensuring this before we can make a pledge."