UK on 11 September terror alert

America is beginning to rebuild after last year's attacks

Britain is on official 11 September terror alert, detectives have revealed.

Scotland Yard says security is being stepped up in the run up to the first anniversary of the attacks on New York and Washington.

Anti-terrorist officers say there is a possibility lone bombers and gunmen unconnected to al-Qaeda could use the date to make a strike on a "world stage".

But police warned the public to be on a state of "alert but not alarm".

London will be ringed by road blocks

Mobile armed road blocks will be in place around London as a first point of defence on the anniversary.

Assistant commissioner David Veness, head of Scotland Yard's anti-terrorist squad, said extra police would patrol the capital, particularly at business institutions, those with links to America, on the Underground and at airports.

BBC home affairs correspondent Margaret Gilmore said Mr Veness had described the terrorist threat being "as high as it has ever been" and the counter operation was "way off the scale".

She said he described the alert as being "bigger than it has ever been, even at the height of the IRA activity in the 1980s".

But he stressed there was no specific threat against Britain and people should go about "business as usual".

We shouldn't just think about the ghastly spectacular attacks that have already taken place, but also the range of possibilities that smaller groups and individuals may use

Assistant commissioner David Veness

He urged them to be vigilant and to look out for anything unusual, which they should report.

Mr Veness said the focus was not just on al-Qaeda and groups associated with it but also individuals who could attack on their own initiative for their own reasons.

He said they "could be drawn to the anniversary because of the publicity it represents.

"We shouldn't just think about the ghastly spectacular attacks that have already taken place, but also the range of possibilities that smaller groups and individuals may use.

"There could be a range of terrorist methodology."

The counter-terrorism operation, he added, was better prepared than ever before following a year of unprecedented effort.

In recent weeks businesses in the City of London have been extensively briefed in preparation for the anniversary.

There has also been close co-operation between Scotland Yard, British Transport Police, the Ministry of Defence Police, the City of London Police, Royal Parks Police and forces nationwide.

Security will be in place for memorial events and Jewish holidays which fall over the 11 September period.