Female suicide bombers: latest al-Qaeda threat

Why was the terror threat to Britain raised on Saturday from "substantial" to "severe"? Two theories have emerged over the weekend to explain the new alert announced by Home Secretary Alan Johnson on the eve of this week's London conferences on Yemen and Afghanistan.

Theory No 1:Al-Qaeda is planning to use women - possibly non-Arab in appearance - as suicide bombers. The Sunday Telegraph reported intelligence sources in the States saying al-Qaeda cells have been training women as suicide bombers to attack western targets including airliners, sports stadiums, ports and power stations.

The women would be travelling on western passports. Some of the women are understood to have been prepared for their missions by the same Yemeni group responsible for the failed Christmas Day plot to blow up an American airliner en route to Detroit.

The paper quotes Richard Clarke, a former White House counter-terrorism adviser, as saying: "They have trained women. There are others who are still out there who have been trained and who are 'clean skins'. ­ That means people who we do not have a record of, people who may not look like al-Qaeda terrorists, who may not be Arabs, and may not be men."

Female suicide bombers have been used in the past by al-Qaeda in Iraq and, most notoriously, by Palestinian groups in Israel. Using them against western targets would be a first. The Sunday Telegraph's security sources believed it was "inevitable" al-Qaeda would eventually turn to using women of a western appearance.

Theory No 2:Al-Qaeda terrorists plan to hijack an Indian airliner and crash it into a British city. Security sources in London fear such an attack after being warned early last week by Indian intelligence of a plot by terrorists linked to al-Qaeda in Pakistan to hijack a plane flying from Mumbai or Delhi.

According to the Sunday Times, British intelligence has long feared such a scenario ever since a reported al-Qaeda plot in 2003 to crash a plane into Heathrow airport. At the time, Prime Minister Tony Blair and Home Secretary David Blunkett ordered tanks to surround Heathrow airport. However, that was seen as a largely symbolic gesture and reports circulated at the time that the tanks weren't even carrying shells.

However, the Indian government is reported to be taking the threat seriously after having captured a suspected Islamic terrorist leader. Passenger screening is being increased at India's international airports and extra armed sky marshals are being deployed on flights operated by Air India and Indian Airlines.

The 'severe' terror alert was announced by the Home Secretary on Saturday because the Joint Terrorism Analysis Centre at MI5 believes an attack is "highly likely". It is the first time the alert level has reached "severe" since last summer. The next level - the highest - is "critical". ·