The world's glaciers are melting faster than at any time since records began, threatening catastrophe for hundreds of millions of people and their eco-systems.

The details are revealed in the latest report from the World Glacier Monitoring Service and will add to growing alarm about the rise in sea levels and increased instances of flooding, avalanches and drought.

Based on historical records and other evidence, the rate at which the glaciers are melting is also thought to be faster that at any time in the past 5,000 years, said Professor Wilfried Haeberli, director of the monitoring service. 'There's no absolute proof, but nevertheless the evidence is strong: this is really extraordinary.'

Experts have been monitoring 30 glaciers around the world for nearly three decades and the most recent figures, for 2006, show the biggest ever 'net loss' of ice. Achim Steiner, head of the UN Environment Programme (UNEP), told The Observer that melting glaciers were now the 'loudest and clearest' warning signal of global warming.

The problem could lead to failing infrastructure, mass migration and even conflict. 'We're talking about something that happens in your and my lifespan. We're not talking about something hypothetical, we're talking about something dramatic in its consequences,' he said

Lester Brown, of the influential US-based Earth Policy Institute, said the problem would have global ramifications, as farmers in China and India struggled to irrigate their crops.

'This is the biggest predictable effect on food security in history as far as I know,' said Brown.

Based on the Intergovernmental Panel on Climate Change's mid-range prediction that global temperatures will rise 2C above their long-term average, UNEP last year warned of further dramatic declines in glaciers by the end of the century.

The revelation that the world's glaciers are in retreat came as Tony Blair began a series of high-level environmental meetings in Japan, China and India as the leader of a new international team charged with securing a global deal on climate change. In a speech yesterday in Chiba, Japan, Blair said that the world now faced catastrophe.

'We have reached the critical moment of decision on climate change,' he said. 'Failure to act now would be deeply and unforgivably irresponsible. The scale of what is needed is so great that the purpose of any global action is not to ameliorate or to make better our carbon dependence; it is to transform the nature of economies and societies in terms of carbon consumption and emissions.

'If the average person in the US is, say, to emit per capita, one-tenth of what they do today and those in the UK or Japan one-fifth, we're not talking of adjustment, we're talking about a revolution.'

The key to that revolution was a vastly increased use of nuclear power across the world, he added.

Blair is also scheduled to meet Yasuo Fukuda, Japan's Prime Minister, and members of the Indian and Chinese governments.