"Action is affordable. Inaction is not," he told the third Greenpeace Business Lecture in central London.

King said levels of the main greenhouse gas, carbon dioxide, were at their highest ever and rising due primarily to the burning of fossil fuels at an unprecedented rate.

King insisted he was optimistic. "We now understand what is happening and therefore what we must do to address this -- global action to reduce carbon emissions is the key," he said.

It was not just cars and the boom in air travel that were the culprits; power stations and factories are pumping out greenhouse gases, and coal and wood remain the main sources of heat and light across large parts of the globe.

King's warning echoed that by fellow British scientist Mike Pilling of Leeds University, speaking at the British Association for the Advancement of Science annual meeting last month.

He warned that the escalation of hurricanes and heat waves would trigger an epidemic of premature deaths and urged governments to come up with a holistic solution rather than trying to pick off culprits one at a time.

His comments came a day after scientists at Britain's Hadley Center revealed that CO2 levels in the atmosphere had risen in the past two years, prompting fears that catastrophic climate change could be out of control.

However, although the CO2 spike had been registered across the world, scientists cautioned that it was too early to tell if it was an anomaly or if climate change had entered a new, explosive phase.

"CO2 levels are up about two ppm (parts per million) in the past two years -- but it would be pushing it to say that it could be the start of runaway global warming," Kim Holmen at the Norwegian Institute for Air Research said.

Environment pressure group Greenpeace said King had sounded an alarm that could not go unnoticed.

"All political and business leaders now have a moral duty to respond to what is clearly an emergency," Greenpeace's executive Stephen Tindale said.

The Kyoto treaty on cutting CO2 emissions by 5.2 percent below 1990 levels by 2012 is expected to come into force within months with crucial Russian backing after the United States refusal to endorse the treaty in 2001 led to years of delays.

But scientists are divided on the treaty's efficacy and environmentalists say it is far too little, too late.