Female terrorist Photo: Alamy

The women, who may have a “non Arab” appearance and be travelling on Western passports, have been prepared for their missions by the Yemeni group responsible for the operation to blow up an airliner over the United States on Christmas Day.

Details of the bombers emerged just hours after British spy chiefs raised the UK threat state to “severe” amid fears that al-Qaeda was planning a wave of attacks against western targets.

Terror experts within the Joint Terrorism Analysis Centre, a unit of MI5, now believe that an attack against the UK is “highly likely”.

US law enforcement agents have been told to be on the lookout for female suicide bombers who may attempt to enter the country.

At least two are believed to be connected to al-Qaeda in the Arabian Peninsula which dispatched underwear bomber Umar Farouk Abdulmutallab who tried unsuccessfully to bring down a transatlantic airliner over Detroit.

“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.”

Security sources said that it was “inevitable” that al-Qaeda would eventually turn to using women with a western appearance to carry out suicide attacks.

Officials said that airliners and all forms of transport could be targeted as well as sports stadiums, ports and power stations.

Although female suicide bombers have been used to carry out attacks by al-Qaeda in Iraq and Palestinian groups in Israel, none are thought to have been used against European or US targets before.

It has also emerged that two male Arab nationals were refused permission to board planes from Heathrow Airport to the United States earlier this month after their names appeared on a US “no fly list”.

The first incident happened last Saturday when British officials refused to allow an Egyptian man permission to fly on American Airlines flight 113 from Heathrow to Miami.

The following day a Saudi Arabian passenger was stopped from boarding United Airlines flight 929 to Chicago. Officials said the man was sent back to Saudi Arabia by British immigration officials.

Both men were denied permission to board the US-bound flights after their names were “flagged” as being linked to potential Islamist terror groups.

Washington officials said the men were part of a surge of people named on its “no fly” list who have attempted to board flights to or in the US in recent days.

In two other cases involving people on the no-fly list, a man in Nairobi, Kenya was kept off a flight last Sunday that would have connected in Amsterdam to Dallas, and a passenger attempting to fly on an American Airlines flight to Los Angeles was stopped in Saint Maarten, in the Caribbean, before he could board a connecting flight to San Juan, Puerto Rico.

American officials say there were two additional incidents, in Minneapolis and in Fort Lauderdale, Florida, in which people on the no-fly list were denied boarding, questioned and then allowed to leave the airport without being detained.

Mr Clarke, who was talking to ABC news in the United states, added: “What we don’t know is whether this is because everyone is doing a better job of enforcing the no-fly list, or because the list has been expanded, or because the terrorists are attempting to probe our security.”

A US Senate Committee on Foreign Relations report has also revealed that around 70 American citizens, some ex-convicts, have disappeared into Yemen and Somalia and pose a Jihadist threat to the US.

The latest revelations follow the decision by the Joint Terrorism Analysis centre (JTAC), which is based at MI5’s headquarters in London, to raise the UK threat state from substantial to severe – a move which indicates that a UK-based attack is now “highly likely”.

The threat is believed to come from al-Qaeda in the Arabian Peninsular. Whitehall sources said that the group had demonstrated that it had both the “intent and capability” to mount an attack against a western target.

The decision to raise the threat level was made following briefings to the Prime Minister by the heads of the security services MI5, MI6 and Government Communication Headquarters (GCHQ), the covert listening agency, and a meeting of the Government’s Cobra emergency committee.

It is understood that the threat state was raised because of the increases in “terrorist chatter” picked up by GCHQ in the last month – a development corroborated by officials on Capitol Hill.

World Foreign ministers are preparing to meet in London this week for two conferences to discuss the threat of terrorism in both Yemen and Afghanistan.

But Whitehall sources were insistent that there was no specific intelligence which suggested that either conference was a potential target.

Announcing the raise in threat level, Alan Johnson said on Friday night that “there is no intelligence to suggest than an attack is imminent”.

He added: “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 to support the police and security services in their continuing efforts to discover, track and disrupt terrorist activity.”

But Patrick Mercer, the Tory chairman of the Commons Counter Terrorism Sub Committee, described the governments warnings as “vacuous”.

He said: “Without any information about what to do or how to do it, then informing us of the threat level is meaningless.”

Gordon Brown warned earlier this week that “a number of terrorist cells are actively trying to attack Britain and other countries.”

His speech to the House of Commons came after he received a briefing on the latest intelligence at a meeting of the Cabinet’s National Security Committee.

Mr Brown said the failed attack over Detroit signalled “the first operation mounted outside Arabia by al-Qaeda in the Arabian Peninsula.”

He also talked about the increased threat from Somalia in east Africa and the Sahel in West Africa, adding that there would be a greater degree of intelligence sharing with foreign countries.

Abdulmutallab, who tried to set off a bomb in his underwear, has warned his FBI interrogators that up to 20 “more like me” may be preparing further attacks.