At the third stroke, the time will be five minutes to the Apocalypse.

The minute hand of the Doomsday Clock, that harbinger of the end of the world, inched forward by two minutes yesterday to reflect the "most perilous period" since the bombing of Hiroshima and Nagasaki.

The clock's keepers, the board of directors of the Bulletin of the Atomic Scientists (BAS), announced the change at simultaneous news conferences in Washington and London. By moving the hand of the clock closer to midnight - the figurative end of civilisation - the BAS is drawing attention to the increasing dangers from the spread of nuclear weapons in a world of violent conflict and to the catastrophic harm from climate change that is unfolding.

Originally intended to warn of the threat of nuclear holocaust, the clock is now almost equally influenced by the looming danger of climate change.

The directors of the BAS said in a statement yesterday: "We stand at the brink of a second nuclear age. Not since the first atomic bombs were dropped on Hiroshima and Nagasaki has the world faced such perilous choices.

"North Korea's recent test of a nuclear weapon, Iran's nuclear ambitions, a renewed US emphasis on the military utility of nuclear weapons, the failure to adequately secure nuclear materials and the continued presence of some 26,000 nuclear weapons in the United States and Russia are symptomatic of a larger failure to solve the problems posed by the most destructive technology on Earth.

"As in past deliberations, we have examined other human-made threats to civilisation. We have concluded the dangers posed by climate change are nearly as dire as those posed by nuclear weapons.

" The effects may be less dramatic in the short term than the destruction that could be wrought by nuclear explosions, but over the next three to four decades climate change could cause drastic harm to the habitats upon which human societies depend for survival."

The decision to move the clock's minute hand forward was made in consultation with a board of sponsors which included 18 Nobel laureates.

Among the sponsors was Stephen Hawking, the theoretical physicist and Lucasian Professor of Mathematics at Cambridge University.

Speaking at the Royal Society in London by way of a computer attached to his wheelchair, the professor said: "Since Hiroshima and Nagasaki, no nuclear weapons have been used in war, though the world has come uncomfortably close to disaster on more than one occasion. But for good luck, we would all be dead.

"We foresee great peril if governments and society do not take action now to render nuclear weapons obsolete and prevent further climate change."

When the clock was established in 1947, the time was set at seven minutes to midnight. Since then it has moved forwards and backwards 17 times, reacting to world events.

In 1953, as the US and Soviet Union tested hydrogen bombs within nine months of each other, the time was set at two minutes to midnight, the closest point it has come to the hour.

It moved back to 17 minutes to midnight, its furthest point from the hour, in 1991 when the two superpowers signed the Strategic Arms Reduction Treaty.

The last time the clock was adjusted was in 2002 when the minute hand was set at seven minutes to the hour.