Researchers who have recently returned from the region found that an area of permafrost spanning a million square kilometers - the size of France and Germany combined - has started to melt for the first time since it formed 11,000 years ago at the end of the last ice age.

The area, which covers the entire sub-Arctic region of western Siberia, is the world's largest frozen peat bog and scientists fear that as it thaws, it will release billions of tonnes of methane, a greenhouse gas 20 times more potent than carbon dioxide, into the atmosphere.

It is a scenario climate scientists have feared since first identifying "tipping points" - delicate thresholds where a slight rise in the Earth's temperature can cause a dramatic change in the environment that itself triggers a far greater increase in global temperatures.

The discovery was made by Sergei Kirpotin at Tomsk State University in western Siberia and Judith Marquand at Oxford University and is reported in New Scientist today.

The researchers found that what was until recently a barren expanse of frozen peat is turning into a broken landscape of mud and lakes, some more than a kilometer across.

Dr Kirpotin told the magazine the situation was an "ecological landslide that is probably irreversible and is undoubtedly connected to climatic warming". He added that the thaw had probably begun in the past three or four years.

Climate scientists yesterday reacted with alarm to the finding, and warned that predictions of future global temperatures would have to be revised upwards.

"When you start messing around with these natural systems, you can end up in situations where it's unstoppable. There are no brakes you can apply," said David Viner, a senior scientist at the Climatic Research Unit at the University of East Anglia.

"This is a big deal because you can't put the permafrost back once it's gone. The causal effect is human activity and it will ramp up temperatures even more than our emissions are doing."

In its last major report in 2001, the intergovernmental panel on climate change predicted a rise in global temperatures of 1.4C-5.8C between 1990 and 2100, but the estimate only takes account of global warming driven by known greenhouse gas emissions.

"These positive feedbacks with landmasses weren't known about then. They had no idea how much they would add to global warming," said Dr Viner.

Western Siberia is heating up faster than anywhere else in the world, having experienced a rise of some 3C in the past 40 years. Scientists are particularly concerned about the permafrost, because as it thaws, it reveals bare ground which warms up more quickly than ice and snow, and so accelerates the rate at which the permafrost thaws.

Siberia's peat bogs have been producing methane since they formed at the end of the last ice age, but most of the gas had been trapped in the permafrost. According to Larry Smith, a hydrologist at the University of California, Los Angeles, the west Siberian peat bog could hold some 70bn tons of methane, a quarter of all of the methane stored in the ground around the world.

The permafrost is likely to take many decades at least to thaw, so the methane locked within it will not be released into the atmosphere in one burst, said Stephen Sitch, a climate scientist at the Met Office's Hadley Center in Exeter.

But calculations by Dr Sitch and his colleagues show that even if methane seeped from the permafrost over the next 100 years, it would add around 700m tons of carbon into the atmosphere each year, roughly the same amount that is released annually from the world's wetlands and agriculture.

It would effectively double atmospheric levels of the gas, leading to a 10% to 25% increase in global warming, he said.

Tony Juniper, director of Friends of the Earth, said the finding was a stark message to politicians to take concerted action on climate change. "We knew at some point we'd get these feedbacks happening that exacerbate global warming, but this could lead to a massive injection of greenhouse gases.

"If we don't take action very soon, we could unleash runaway global warming that will be beyond our control and it will lead to social, economic and environmental devastation worldwide," he said. "There's still time to take action, but not much.

"The assumption has been that we wouldn't see these kinds of changes until the world is a little warmer, but this suggests we're running out of time."

In May this year, another group of researchers reported signs that global warming was damaging the permafrost. Katey Walter of the University of Alaska, Fairbanks, told a meeting of the Arctic Research Consortium of the US that her team had found methane hotspots in eastern Siberia. At the hotspots, methane was bubbling to the surface of the permafrost so quickly that it was preventing the surface from freezing over.

Last month, some of the world's worst air polluters, including the US and Australia, announced a partnership to cut greenhouse gas emissions through the use of new technologies.

The deal came after Tony Blair struggled at the G8 summit to get the US president, George Bush, to commit to any concerted action on climate change and has been heavily criticized for setting no targets for reductions in greenhouse gas emissions.