London – Britain’s terrorist threat level was raised tonight from “substantial” to “severe” – meaning that counter-terrorism agencies believe an attack is “highly likely”.

The measure was approved at a meeting of the Government’s Cobra emergency committee and announced by Alan Johnson, the Home Secretary.

The Times UK understands that the decision to raise the threat level is connected to the conference on Afghanistan taking place at Lancaster Gate, London, next Thursday.

Sources said there had been intensive discussions throughout the day relating to intelligence suggesting a possible attempted “spectacular” by an al-Qaeda affiliated group.

But the shift was also described by one source as “precautionary” rather than rooted in any firm information that an identified terror cell was plotting an attack.

Mr Johnson said: “The Joint Terrorism Analysis Centre has today raised the threat to the UK from international terrorism from substantial to severe. This means that a terrorist attack is highly likely, but I should stress that there is no intelligence to suggest than an attack is imminent. JTAC keeps the threat level under constant review makes its judgments based on a broad range of factors, including the intent and capabilities of international terrorist groups in the UK and overseas.

“In his statement to Parliament on security and counter terrorism earlier this week, the Prime Minister said that the first and most important duty of government is the protection and security of the British people. We still face a real and serious threat to the UK from international terrorism so I would urge the public to remain vigilant and carry on reporting suspicious events to the appropriate authorities and to support the police and security services in their continuing efforts to discover, track and disrupt terrorist activity.”

(Source: Times Online UK / YWN-112)