An asteroid nicknamed "The Rock" that could be big enough to devastate much of the UK is making its closest approach to Earth in 400 years.

The "potentially hazardous" asteroid 2014 JO25 will fly past Earth at 33 metres per second at a distance of just over a million miles - far enough away to be safe but uncomfortably close in astronomical terms.

Astronomers will closely monitor The Rock as it comes within 4.6 lunar distances from Earth at 1.24pm UK time on Wednesday.

Estimates of its size vary but experts put it between 650 metres (2,132ft) and 1.4 kilometres (0.87 miles) in length.

An asteroid a mile wide hitting the Earth would unleash as much energy as around 1,000 atom bombs like the one dropped on Hiroshima in 1945.

The blast would completely destroy a city the size of London or New York and cause extensive damage for hundreds of miles.

Scientists first learned of The Rock three years ago when it was spotted by the Catalina Sky Survey, a US programme that uses two telescopes in Arizona to detect potentially dangerous near-Earth objects (NEOs).

No other asteroid of comparable size has come this close to the Earth in the last 13 years.

Astronomers believe they know about 90% of potentially hazardous asteroids bigger than 1,000 metres across.

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Smaller objects that are still large enough to cause a huge amount of destruction are a more worrying problem.

Only 30% of 140 metre (459ft)-sized NEOs have been detected and less than 1% of those in the 30-metre range.

Even a 30 metre-wide asteroid could cause significant damage to a major city.

The asteroid that is thought to have killed off the dinosaurs 66 million years ago was between six and eight miles across.

While not big enough to cause an extinction level event, the effect of an object the size of The Rock hitting the Earth could be catastrophic on a regional scale.

Scientists at the University of Colorado, US, have calculated that it would take a 60 mile-wide object to obliterate the human race.

In September 2004, the 3.1-mile wide asteroid Toutatis approached within four lunar distances of the Earth.

Robotic telescope service Slooh, which allows members of the public to be online astronomers, is tracking the asteroid from its flagship observatory on the Canary Islands. Its coverage is accessible via the website.

A Slooh spokesman said: "The Rock's close approach on the 19th will not bring it on a collision course with the planet, as it passes at a safe distance, but its size, proximity, and speed are an alarming reminder of just how close these destructive chunks of space debris come to Earth on an almost daily basis."