George W Bush has approved a plan to shoot down a spy satellite that could crash to Earth.

The satellite was launched in December 2006 and carries a sophisticated and secret imaging sensor. It lost power soon after launch and the central computer failed, leaving it out of control in orbit. It is expected to enter the Earth's atmosphere in the first week of March.

There are fears the toxic rocket fuel hydrazine could threaten human life if it crash lands so the President has agreed to fire a tactical missile in an effort to blow the satellite up before it causes any damage.

Deputy National Security Advisor James Jeffries said: "In particular consideration of the question of saving or reducing injury to human life, the President, on the recommendation of his national and homeland security teams, directed the Department of Defence to carry out the intercept."

The missile is likely to be launched from a US Navy ship within ten days, but exact details of the plan have not been revealed.

General James Cartwright, vice chairman of the Joint Chiefs of Staff, said: "We'll use one missile with two backups. We'll have three ships on station, but it'll be one shot."

General Cartwright did not reveal the odds of success but he admitted the launch is unprecedented.

He added: "We believe that the window for this activity will start here in the next three or four days and will be open for about maybe as many as seven or eight days.

"Much of this depends on the heating of the atmosphere. So we're trying to build where would the best position be from the Earth to launch a missile to intercept that would drive this down into the ocean. Our objective is to get rid of the hydrazine and have this fall in the ocean."

A Standard Missile 3 will be fired before the satellite re-enters the atmosphere because disturbances caused by the gases surrounding the Earth will make it almost impossible to hit after that.

The satellite weighs two tons and around half of that will burn up if it re-enters the atmosphere but the rest would hit the Earth, scattering debris over an area covering hundreds of miles.

General Cartwright said officials had decided it was better to attempt the interception than not.

There is a back-up plan to aim for a direct hit on the satellite's fuel tank to limit the amount of flammable material that returns to Earth.

Nasa is concerned the satellite could leave huge chunks of space debris once it has been hit by a missile and they are concerned that flotsam could hit the international space station.

Nasa administrator Michael Griffin said: "There are good times to conduct the intercept, and poor times to conduct the intercept based on the positioning of the station and I and my colleagues will work together to make sure that, if possible, we pick one of the good times."

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