Britain's security services have been put on their second highest state of alert amid intelligence of a possible al-Qaeda attack, the BBC has learned.
Previous alerts have led to roadside checks by police
The internal "severe general" alert is said to be unconnected to US President Bush's forthcoming UK visit.
It follows warnings about plans by al-Qaeda supporters from North Africa.
The alert means security will be extra tight around potential targets. Sources say no attack is imminent, and there is no intelligence of a specific target.
The warning comes as police plan an "unprecedented" security operation for US President Bush's state visit to the UK next week.
The internal alerts are for the security services only, including the police, the army, MI6 and MI5, and not usually made public.
They were introduced after the Bali bombings to help those involved in the fight against terrorism respond to changing circumstances.
The UK is officially on high alert, but within this is a tier of internal and usually secret levels - which have gone from "substantial" to "severe general".
BBC home affairs correspondent Margaret Gilmore said it was "rare" to be put on this higher alert, but the public should not necessarily be concerned.
"These are warnings very much for the security services, these are not warnings aimed at the public because there is not a lot the public can do about them.
"They don't want to scaremonger, they don't want people to get worried.
"This is a subtle change. It means they will be more vigilant, but it doesn't mean there is anything the public can do, and it doesn't mean there is a missile incoming or anything like that."
'No specific threat'
She added: "Obviously it's a further complication in the security nightmare surrounding the Bush visit, but it is not connected to that."
The Home Office has refused to comment, saying it never discusses threat levels unless there is a specific threat.
A spokeswoman said: "If there's a specific threat the government would not
hesitate to issue an alert."
About 5,000 Metropolitan Police officers and all its armed units will be on duty next week to ensure Mr Bush's controversial state visit goes smoothly.
"The bubble" - a ring of 700 of Mr Bush's own secret service agents - will
also surround the president amid fears of terror attacks.
Talks with protesters on demonstration routes are still ongoing, though they
will be banned from going down Whitehall and to the House of Parliament.