Samantha Lewthwaite pictured in 2005 with her Jamaican husband and fellow 'Muslim convert', Jamaican-born Jermaine Lindsay, who was set up by MI5 in the 7/7 London Bombings.
Police in Kenya have issued an arrest warrant for a woman using the name Natalie Faye Webb, and carrying a forged South African passport.
Helped by officers from Scotland Yard who flew out to offer advice, they have published her picture to alert the public.
But investigators say the woman has three separate identities and one is that of Samantha Lewthwaite, who was married to
Jermaine Lindsay, one of the 7/7 bombers.
Lindsay killed 26 people when he blew himself up on the Piccadilly Line between King's Cross and Russell Square in July 2005.
Lewthwaite, from Aylesbury, Bucks, who converted to Islam at the age of 15 and married Lindsay in 2002, is said to be travelling with her three children.
She is suspected of having links to a terrorist cell planning an attack in Kenya after their troops crossed the border to fight al-Shabaab, an al-Qaeda-linked group in Somalia.
The militant group al-Shabaab, which is currently engaged in a guerrilla war against Kenyan security forces in Somalia and has links to al-Qaeda, is believed to be behind the cell. Up to 40 Britons are fighting with al-Shabaab.
"I can give no details, but suffice it to say that we believe she is not a small fish," a senior police source in Nairobi. "She is among several Britons that our intelligence service is aware of in relation to terrorists' plans to attack us."
Kenyan police claimed to be closing in on the woman at that stage but she has so far evaded capture.
Miss Lewthwaite's father Andy told the Times he did not believe she would be involved in a terror plot after travelling with her three children.
"I haven't spoken to her for a long time," he said. "I don't know if she's in this country or where she is."
She entered the country to meet with other terror suspects in Mombasa using a South African passport under the name of Natalie Faye Webb, aged 26.
Miss Webb was reported as having connections to South Africa, her mother's native country, but was said to have never visited Kenya.
Her father said she was a victim of identity theft.
"We have been liaising with officers from the anti-terrorism unit at New Scotland Yard to find out how this has come about," he said.
Police are also hunting for a man called Habib Ghani, from Hounslow, West London, who left Britain several years ago to live in Africa.
They are said to be connected to Jermaine Grant, 29, a Muslim convert of Jamaican origin from Newham, East London, who has appeared in court in Kenya on charges of possessing explosives and preparing an attack.
He was arrested in the coastal town of Mombasa in December, in an apartment where police also found bomb-making chemicals, batteries and an electric switch.
Security sources in Nairobi said that when he was detained, Grant had plans for hotels and restaurants in the Kenyan capital.
He was charged alongside Kenyans Fouad Abubakar Manswab, Warda Breik Islam and Frank Ngala. Grant had apparently married Islam days before he was arrested.
The charge sheet showed that they were allegedly found in possession of bomb-making materials including batteries, wire, ammonium nitrate, lead nitrate, acetone and hydrogen peroxide.
"On 20 December ... jointly with others not before the court, they conspired to improvise an explosive with intent to cause loss of human lives and harm to innocent citizens," it said.
Grant has already been sentenced to three years in prison for using false documents that claimed he was a Canadian called Peter Joseph.
Grant was reportedly radicalised when previously held for non-terrorist-related offences in Feltham young offenders institution in London.
Media reports in Kenya said Grant had also been arrested with two others in 2008 while trying to sneak into Somalia disguised as women.
Miss Lewthwaite, 27, was seven months pregnant with their second child when her husband launched his 7/7 attacks, although it emerged at the inquests last year that Lindsay was cheating on her.
She had taken the name Sherafiyah after converting and met Lindsay in 2002 over the internet.
She initially denied Lindsay's involvement in the 7/7 attacks until authorities produced forensic evidence to confirm his identity, although she later said she "abhorred" the attacks and that her husband's mind had been poisoned by radicals.
In August 2009, she gave birth to a baby she named Abdur-Rahman Faheem Jamal - the same surname as the two other children she fathered with Lindsay.
However it was unclear who the father was because there was no name on the birth certificate and she refused to reveal his identity.
A family insider said they were told the father was a Muslim Moroccan from Birmingham.
Besides you don't get a lot of sympathy or outrage from harassing a young mother with three children. So obviously they can't set her up without people asking questions, so they have to 'protect' her.