LONDON - Britain has raised terror threat level from substantial to severe after fears of a terrorist attack during two major summits on Afghanistan and Yemen taking place at Lancaster House London next Thursday. US Secretary of State Hillary Clinton and other leading foreign ministers are due in London next week. Also at the meetings will be Afghan President Hamid Karzai and the UN Secretary-General Ban-Ki Moon. British Home Secretary Alan Johnson announced the shift on Friday night. The new level denotes that an attack is 'highly likely and is the second highest on a five-step scale. Intelligence sources said the decision followed a 'non-specific warning from the CIA that Britain is facing an increased threat from Al-Qaeda. The terror threat has not been at this level since July last year. The British Home Secretary insisted that there was no intelligence to suggest an attack is imminent, but said the public should be 'more aware. The UKs official terror threat was at the severe level for four years after the July 7 bombings in London in 2005 but was downgraded last summer. Alan 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 that an attack is imminent, he added. 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, he said and added, We still face a real and serious threat 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. A spokesman from the Metropolitan Police said the service would review its contingency planning and policing tactics following the announcement. People who live, work and visit the capital will continue to see a wide selection of policing tactics being used, he said. Prof Richard Bonney, a leading expert, believes that the official level has gone from substantial to severe as the arrival of the US Secretary of State is due in London next week. Prof Bonney, from the University of Leicester, said: I suppose that Hillary Clintons arrival is the precipitant; but the Afghan conference is a high profile target, given the number of interested players. (Her visit to London) is a very likely, plausible threat. Its a high profile target for some groups, those who seek to disrupt any sort of future peace process in Afghanistan in particular, he added. Prof Bonney further said unless there was some other associated threat that intelligence agencies had been made aware of, it was likely the level will drop back to 'substantial shortly after the conference. It may be maintained for a period afterwards but unless theres some other associated threat which we dont know about its unlikely it would continue for more than a month, he said. He further said that it was unlikely Al-Qaeda would be considering an attack, but affiliated groups may be interested in the conference as a target.