"The Joint Terrorism Analysis Center 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," he said in a statement.

"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," the statement said.

Britain hosts an international conference on Afghanistan on January 28 that London says may set a timetable for transferring responsibility for some areas to Afghan control.

Those present will include Afghan President Hamid Karzai, U.N. Secretary-General Ban Ki-moon, NATO Secretary-General Anders Fogh Rasmussen, U.S. Secretary of State Hillary Clinton and foreign ministers of Afghanistan's other main partners.

It will be preceded by a January 27 meeting on Yemen, which declared war on al Qaeda last week under pressure for a crackdown on the global militant group after its Yemen-based wing said it was behind an attempt on December 25 to blow up a U.S. passenger jet.

The West and neighboring Saudi Arabia fear Yemen could become a failed state, allowing al Qaeda to use the country as a launchpad for further international attacks.

The meeting brings together foreign ministers of Yemen's main development partners.

A Home Office advisory said people the threat level meant people should remain on alert for danger, including looking or suspicious bags on public transport. But it added that they should go ahead with life as normal as the risk of being caught up in an attack was very low.

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