A policeman stopped for speeding while driving Home Secretary Jack Straw in his official car will not face prosecution.

The announcement from Avon and Somerset Police comes four months after Mr Straw's car was stopped on the M5 motorway.

Mr Straw's unmarked Jaguar, driven by his armed bodyguard, was said to have been doing more than 100mph when it was pulled over on the southbound carriageway.

The incident happened as the car was being driven along the M5 by a special branch officer en route to a Labour Party meeting in Exeter on 9 July.

Police allowed the car to continue after taking the name of the Special Branch officer, who has not been named.

'Insufficient evidence'

In a statement police said the driver had "believed that there was a potential risk to the home secretary's security, and decided to use speed to create distance between the risk and the home secretary, in line with his training".

Police said the Chief Crown Prosecutor for Avon and Somerset had reviewed all the evidence and advised there was "insufficient evidence for a realistic prospect of conviction against the Metropolitan Police Service driver".

Downing Street said at the time it would not intervene on the driver's behalf.

A spokesman for Prime Minister Tony Blair said: "The law provides for speed limits, the police have the discretion to apply them as they see fit".

'Lead by example'

Mr Straw was criticised after the incident by the Police Federation.

Metropolitan Police Federation chairman Glen Smyth said at the time: "The home secretary needs to lead by example and this is not the example to set."

The home secretary has been vocal in his support for "zero tolerance" towards speeding motorists, and the government has been active in installing new speed cameras around the country.

Under court guidelines any motorist going over 100mph can be banned from driving and fined up to �800.