Despite being the biggest global contributor to so-called "adaptation" funds, the UK is still paying only a tiny fraction of the contribution it should, according to Oxfam.

It pointed out that the total pledged by all rich countries was only about half the £178 million allocated by ministers for cooling systems to climate-change proof the London Underground.

Ahead of next month's G8 summit in Germany, Oxfam calculated that the group of richest nations owed 80% of the £25.2 billion needed to help the poorest deal with the impact.

Experts say 250 million people in Africa could face water shortages in Africa by 2020 as a result of global warming at the same time as billions are put at risk of flooding in Asia.

Britain's share of the cost - calculated by Oxfam on the basis of emissions from 1992 to 2003 and ability to pay - is more than 5%, or £1.3 billion.

So far it has contributed £20 million, more than any other country including the USA - which should be responsible, according to the report, for a massive 44% of the cost.

"Rich countries are already making huge investments at home to adapt to climate change - so they know the scale of the problem. But they are stalling when it comes to providing money for poorer countries to do the same," said Kate Raworth, Oxfam senior researcher and author of the report.

"Developing countries cannot and should not be expected to foot the bill for the impact of rich countries' emissions. Justice demands that rich countries pay for the harm already being caused to those who are least responsible for the problem.

"But it's also crucial in building the trust between nations essential for the success of any truly global agreement to tackle climate change."