Yemen meeting caused switch to 'severe' terror alert

The terror threat to the UK was raised to "severe" after intelligence suggested this week's London conferences on Yemen and Afghanistan could be used by al-Qa'ida as an opportunity to strike at the West, Whitehall sources said yesterday.

The Home Secretary, Alan Johnson, announced that Britain is on its second-highest alert level – meaning an attack is considered "highly likely" – on Friday evening amid concerns in London and Washington that al-Qa'ida is planning a renewed push against US and UK interests.

Other, anonymous, sources – keen for levels of public awareness, or anxiety, to be raised – gave different accounts. The Sunday Telegraph said that a group of female terrorists, who may have a "non-Arab" appearance, had been trained to attack western targets. The Sunday Times reported MI5 fears that Islamic terrorists planned to hijack an Indian airliner and crash it into a British city.

Gordon Brown and David Miliband will host a counter-terrorism conference on Yemen on Wednesday, followed by a major international meeting on Afghanistan, attended by President Hamid Karzai and US Secretary of State Hillary Clinton, when measures to bolster Kabul's fight against the Taliban will be unveiled.

US and British intelligence services are also on heightened alert ahead of the trial in New York of Khalid Sheikh Mohammed, the self-confessed mastermind of the 9/11 attacks. The escalation of the threat follows the failed attempt to blow up a Northwest Airlines plane to Detroit on Christmas Day. Al-Qa'ida in the Arabian Peninsula, based in Yemen, has claimed responsibility. There have been several attempted strikes against US interests since early November, including the killing of CIA agents in an attack on a US base in Afghanistan last month.

The Home Secretary said yesterday that there was no intelligence to suggest an attack was imminent but urged people to be vigilant. He insisted the heightened state of alert was not specifically linked to the Christmas Day plot. But a Whitehall source said: "The conferences in London on Afghanistan and Yemen provide a focus for al- Qa'ida to do something against British and US interests. It could be a US airliner leaving the UK, or US interests in the UK, or US interests abroad."

Last week the Prime Minister announced new counter-terror 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. Mr Johnson said yesterday: "We still face a real and serious threat to the UK from international terrorism, so I would urge the public to remain vigilant and carry on reporting suspicious events to the appropriate authorities and support police and security services."

The threat level, assessed by the Joint Terrorism Analysis Centre based in MI5, was first made public in 2006. It was downgraded from "severe" to "substantial" last summer. The threat level has temporarily reached "critical" – meaning an attack is imminent – twice since being made public.