The first thought, notes the Times of London, is that the increase in the threat level — which had been at the middle level, “substantial” — is tied to Thursday’s upcoming conference with various foreign ministers on the way forward in the Afghanistan war. There’s also the view that this is a general reaction to intelligence gleaned — perhaps with renewed vigor — after al-Qaida attempted the Christmas Day bombing of a U.S.-bound airliner by a suspect who was schooled in Britain (and tried — and was denied — re-entry into the UK months before packing the bomb in his undies and boarding Flight 253 from Amsterdam to Detroit). Here’s what the Times is reporting:

“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.’”

Here’s more on the threat level system in the UK from the Home Office.

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