Britain’s terrorist threat level has been raised 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 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.

From MI5 The Joint Terrorism Analysis Centre (JTAC) has changed the UK threat level from international terrorism from SUBSTANTIAL to SEVERE. The current threat level means that the threat of a terrorist attack is considered to be highly likely.

The Home Secretary said in a statement:

“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.”