UK terrorthreat levelincreased to 'severe'

attack is imminent. “JTAC keeps the threat level under constant review and makes its judgments based on a broad range of factors, including the intent and capabilities of international terrorist groups in the UK and overseas.”

Mr Johnson added: “The highest security alert is critical and that means an attack is imminent, and we are not at that level.”

He said more measures had been taken to protect aviation since the alleged failed bomb attack on a plane at Detroit at Christmas.

“We have a very adept and very focused counter-terrorism facility in this country and people should be reassured by that.”

He said that moving to the different threat level meant people had to be more alert, but there was absolutely no intelligence to suggest that an attack was imminent.

Mr Johnson said that yesterday’s decision was not specifically linked to the Christmas Day plane bombing attempt or any other incident.

The Detroit incident has moved the focus of the fight against terror onto the Yemen, where suspect Umar Farouk Abdulmutallab has reportedly claimed he was supplied with explosives.

Following the attack, the Yemeni government has stepped up a US-backed military operation against al-Qaida sympathisers based in remote areas of the Middle Eastern country. And Prime Minister Gordon Brown has called an international counter-terrorism meeting on Yemen in London for next Wednesday.

A security review conducted in the wake of the failed attack led to Mr Brown’s announcement on Wednesday of new measures, including a suspension of direct flights between the UK and Yemen and a “no-fly list” to prevent people with suspected militant links from travelling to Britain.