A team of non-Arabic female suicide bombers has been sent to attack targets in the West, it is feared.

Security officials have been warned to be on the look-out for the women, carrying Western passports, who are believed to have escaped an attack on an Al Qaeda terror camp in Yemen.

Former White House counter-terrorism adviser Richard Clarke said: '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. They have trained women.'

A U.S. air strike on Christmas Eve is believed to have killed many suicide bombers being trained in Yemen, but some are thought to have escaped.

Umar Farouk Abdulmutallab, the former British student accused of trying to detonate a bomb on a plane over Detroit on Christmas Day, told the FBI several other people trained with him in Yemen.

Female suicide bombers have previously struck in Iraq and Israel but security sources regard the use of western-looking women for Al Qaeda attacks on the U.S. or Europe as 'inevitable'.

The warning came as Britain's security threat was raised to 'severe' - the second-highest level - ahead of two London conferences on Afghanistan and Yemen.

They will be attended by leading figures from 60 countries, including U.S. Secretary of State Hillary Clinton

The high level of alert saw a check-in area at Manchester Airport evacuated yesterday after containers of white powder were found in the hand luggage of a Heathrow-bound passenger.

The check-in desks and security screening zones were closed for several hours before police and fire services established that the powder was harmless.

The threat level was raised after MI5's Joint Terrorism Analysis Centre noted an increase in so-called 'chatter' suggesting an attack was more likely.

It was also partly prompted by warnings that Al Qaeda terrorists may be planning to hijack an Indian passenger airliner from Mumbai or Delhi and crash it into a city.

The warnings from the Indian Intelligence Bureau - following the interrogation of suspect Islamist terror leader Amjad Khwaja - are said to have raised fears in London that the terrorists might have a British target in their sights.

Screening and frisking of passengers has been increased in Indian airports.

Foreign Secretary David Miliband said it would 'very stupid' of him to comment on the intelligence behind the change in threat level.

'The fact is though that these people will stop at nothing,' he told the BBC's The Andrew Marr Show this morning

'They will try every trick in the book, they will use advanced technology, they will use all the mechanisms of open society that we depend on for their own terrible purposes.

'And they will try to strike Christians, Muslims, Jews randomly.'

He added that the Government had a responsibility to keep the terrorist threat to the UK 'under very careful scrutiny'.

However, Home Secretary Alan Johnson said there was 'no intelligence to suggest that an attack is imminent'.

Britain is also introducing 'no-fly lists' to prevent people assessed as a security threat from boarding aircraft, while direct flights between Yemen and the UK have been suspended.

Two Arab men were barred from boarding U.S.-bound planes at Heathrow earlier this month after their names were flagged up as potentially being linked to terrorism, it has emerged.

British officials last Saturday refused to let an Egyptian fly on an American Airlines flight to Miami and the next day stopped a Saudi passenger from boarding a United Airlines flight to Chicago.

Two other would-be passengers heading for the U.S. have been barred by officials in other countries, and two similar incidents have happened on internal U.S. flights.

Mr Clarke said it was unclear whether these incidents indicated an increased threat or simply increased vigilance.

Britain's terror alert level was downgraded last July after standing at 'severe' for four years after the July 7 2005 bombings in London.