Worried astronomers warned 1999 FN53, which is an eighth of the size of Mount Everest, will skim the Earth in TWO DAYS.
A collision would be nothing short of catastrophic triggering mass destruction, earthquakes and global extinction.
The monster is more than TEN TIMES bigger than other meteorites currently visible on NASA’s Near Earth Object radar.
It is also double the size of the gargantuan 2014-YB35 which had astronomers around the world watching the skies in March.
Experts warn a collision would trigger an explosion similar to millions of megatons of TNT and would be capable of killing 1.5 billion people.
It would be far more destructive than the 1908 Tunguska Event which saw a 50-metre lump of extraterrestrial rock crash into Siberia.
It flattened around 80 million trees and sent a shock wave across Russia measuring five on the Richter scale.
The event is held by scientists as a benchmark for the catastrophic consequence of an asteroid impact with earth.
The gigantic lump of rock is travelling faster than 30,000 mph and will brush terrifyingly close to Earth on Thursday.
He said: “People are concerned about an impact from a very large asteroid, and the impact of something of this scale would be nothing short of global.
“It is certainly one of the biggest on the radar, and much bigger than the Tunguska asteroid which was one of the most significant in history.
“This is in a completely different ball park, we are talking about millions of megatons of energy, vastly more than was released in Hiroshima.
“It would undoubtedly lead to the deaths of around 1.5 billion people, we are looking at a mass extinction of humanity.
“To understand the impact of something on this scale, you would have to look to the science fiction writers, it is incomprehensible.”
Professor Napier said: “It is a bit like shooting through a key hole.
“All being well this one is far enough away not to do us any harm, but people are concerned because you just don’t know.”
He added: “This would allow unrestricted sunlight hit the Earth, the sky would heat up becoming strong enough to burn vegetation.
“It would also put a lot of water into the stratosphere with these effects ultimately leading to a mass extinction.”
NASA’s Near Earth Object Programme puts the enormous lump of rock on course to pass within six million miles of Earth on May 14.
In astronomical terms this is a tiny distance and close enough to prompt astronomers to keep an eye on it until is passes safely.
Its exact size is still unclear though it is estimated to be between 580 metres and 1.3km wide – most likely around 680 metres.
In a statement NASA said: “1999 FN53 was discovered on 1999 Mar 31 by the Lowell Observatory Near-Earth Object Search (LONEOS).
“It has an absolute magnitude of 18.3 suggesting a diameter within a factor of two of 680 metres but otherwise its physical properties are poorly known.
“The asteroid will approach Earth…on 2015 May 14.”