A senior investigating officer in the Kenyan Police Force, who asked not to be named said: “Our anti-terrorism officers had been watching her, they were acting on intelligence I cannot say from where, since the beginning of December. . . When the officers decided to arrest her, she had already gone when they went to raid her house. . . I think we can say that seeing him [Grant] there in the dock gave that lady a warning that we were also watching her.”

Lewthwaite, the daughter of a British soldier who grew up in the Home Counties, converted to Islam as a teenager and married Lindsay in 2002. (I think there is doubt about whether they were legally married or merely co-habiting following an Islamic nikah) She was pregnant with their second child in July 2005 when he blew himself up on the Piccadilly Line tube killing 26 people. In 2009 she gave birth to a third child, but has had no contact with her family since and is thought to have left the UK last year.

Police fear she and Ghani, who was renting room in a similar property around half a mile away, were planning to unleash a terror attack as the tourist season got underway. But she was afforded so much warning by the Kenyan authorities that she was able to remove large amounts of potentially crucial evidence. All detectives found when they raided her apartment in January was a child’s cot and some documents which investigators are now examining for any potential leads. The 28-year-old had flushed a number mobile phone SIM cards down the lavatory which were retrieved and are now being analysed by technical experts.

Her alleged accomplice, Ghani, grew up in Hounslow, west London and worshipped at the same mosque as the first British suicide bomber Asif Hanif, who blew himself up in a bar in Israel in 2003. When police searched his apartment they discovered ammunition for AK47 assault rifles as well as a number of detonators. Witnesses recalled how the Middle Eastern looking man, who spoke English and Swahili, was regularly visited at the property by a white woman wearing a traditional Muslim hijab.

Kenyan police sources said they believed the pair were not working on the same targets as Grant, but have not ruled out that they were known to one another. The source said: “That man [Grant] was doing something else, but the truth is he was one step away from a bomb. There was everything ready, waiting only for assembly.”

The woman is believed to have arrived in Kenya on a false South African passport in the name of Natalie Faye Webb. The family of Miss Webb, who lives in Essex and has no connection with the alleged plot, but has dual British and South African nationality, said they have no idea how here identity came to be stolen.

A spokesman for the Kenyan Government would not confirm that the woman they were searching for was Miss Lewthwaite insisting that she could be travelling under a false identity. But he added: "We believe she is a collaborator with terrorists not a planner. Our understanding is that she was working with people here, al Qaeda or al-Shabaab people. She is a very big sympathiser with those people. She was not going to carry out an attack but she helped to fund raise, helped in the acquisition of weapons, hiding people, transporting people, that kind of thing."