Scientists say the Doomsday Clock has moved closer to midnight than it has ever been and is now just 100 seconds away from striking 12.
The clock, which serves as a metaphor for global apocalypse, was moved forward by 20 seconds.
The announcement from the clock's keepers means the perceived threat is now more severe than it was last year, and in 1953, when it was two minutes away.
Former president Mary Robinson, chairwoman of The Elders and former United Nations high commissioner for human rights, described the announcement as a "solemn occasion".
She said: "We ask world leaders to join us in 2020 as we work to pull humanity back from the brink.
"The Doomsday Clock now stands at 100 seconds to midnight, the most dangerous situation that humanity has ever faced.
"Now is the time to come together - to unite and to act."
Rachel Bronson, president and chief executive of the Bulletin of the Atomic Scientists, which sets the reading, said: "The world has entered into the realm of a two-minute warning, a period when danger is high and the margin for error low.
"It is 100 seconds to midnight. We are now expressing how close the world is to catastrophe in seconds - not hours, or even minutes.
"It is the closest to Doomsday we have ever been in the history of the Doomsday Clock.
"We now face a true emergency - an absolutely unacceptable state of world affairs that has eliminated any margin for error or further delay."
A statement from the bulletin said: "Humanity continues to face two simultaneous existential dangers - nuclear war and climate change - that are compounded by a threat multiplier, cyber-enabled information warfare, that undercuts society's ability to respond.
"The international security situation is dire, not just because these threats exist, but because world leaders have allowed the international political infrastructure for managing them to erode."
While the clock did not move in 2019, its minute hand was set forward in 2018 by 30 seconds, to two minutes before midnight.
It was adjusted in 2017 to two-and-a-half minutes to midnight from its previous setting of three minutes to midnight.
The countdown was established in 1947 by experts from The Bulletin of the Atomic Scientists who were working on the Manhattan Project to design and build the first atomic bomb.
The bulletin is an independent non-profit organisation run by some of the world's most eminent scientists.
It was originally intended to warn of the threat of nuclear armageddon.
But the Doomsday Clock also takes into account the likelihood of other emerging threats such as climate change, and advances in biotechnology and artificial intelligence.