An asteroid about two-thirds of a mile wide could hit the earth in 11 years time.

In fact, scientists have given a precise date for the possible collision - March 21, 2014.

It has been classified as ''an event meriting careful monitoring'' by astronomers.

But they say the probability of impact with the Earth is one in 909,000.

The asteroid, known as 2003 QQ47, will be observable from Earth for the next two months and astronomers will study it over this period. If the 2,600 million-ton mass hits the Earth, it would have the force of 350,000 mega tonnes - about eight million times more powerful than the bomb dropped at Hiroshima at the end of the Second World War.

But the current orbit calculations are based on only 51 observations during a seven-day period.

Dr Alan Fitzsimmons, of Queen's University, Belfast, one of the expert team advising the UK NEO (Near Earth Objects) Information Centre, based in Leicester, said there was no cause for concern. He said: ''There is some uncertainty about where it is going.

''In all probability, within the next month we will know its future orbit with an accuracy which will mean we will be able to rule out impact."

Meanwhile, bookmakers William Hill said it was ''happy'' to take bets at odds of 909,000/1 that the asteroid would hit the earth, wiping out life at the same time.