The keepers of the so-called Doomsday Clock, which counts down to Armageddon, today moved its hands closer to midnight for the first time in four years to reflect the growing threats to mankind from nuclear proliferation and climate change.

In a ceremony hosted by the British theoretical physicist Stephen Hawking, the minute hand was moved forwards by two minutes to stand at five minutes to midnight - the closest it has come to midnight since the Cold War arms race of the 1980s.

The decision by the directors of the Bulletin of the the Atomic Scientists was made in consultation with the Bulletin's Board of Sponsors, which includes 18 Nobel laureates.

The BAS said that the world faced its most critical choices since the bombing of Hiroshima and Nagasaki at the end of the Second World War.

"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," it said.

"North Korea’s recent test of a nuclear weapon, Iran’s nuclear ambitions, a renewed 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 failure to solve the problems posed by the most destructive technology on Earth."

The BAS statement continues: "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 irremediable harm to the habitats upon which human societies depend for survival."

Professor Hawking, the Cambridge physicist best known for his book A Brief History of Time, said: "As scientists, we understand the dangers of nuclear weapons and their devastating effects, and we are learning how human activities and technologies are affecting climate systems in ways that may forever change life on Earth. As citizens of the world, we have a duty to alert the public to the unnecessary risks that we live with every day, and to the perils we foresee if governments and societies do not take action now to render nuclear weapons obsolete and to prevent further climate change."

Since 1947 the clock, with midnight representing nuclear apocalypse, has appeared on the cover of the Bulletin magazine, which was founded by University of Chicago physicists alarmed about the dangers of the nuclear age.

The minute hand was last moved in February 2002, when it was pushed forward by two minutes, to seven minutes to midnight, after the US announced plans to withdraw from the Anti-Ballistic Missile treaty.

Today's shift forward marks a rapid decline in international nuclear relations since the 1990s, when the minute hand stood at 17 and 14 minutes to midnight in a post-Cold War thaw.

The closest the clock has come to midnight is just two minutes away. That was in 1953, when the US and the Soviet Union tested thermonuclear devices within nine months of each other.

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