Westminster terrorist Khalid Masood converted his eldest daughter to Islam,
The 52-year-old jihadi convinced his child, now 24, to change her name and wear a full face burka, a close family friend has confirmed.
It sparked a bitter fight with his former girlfriend, successful businesswoman Jane Harvey, who separated from him years earlier when he was jailed for GBH.
Three years ago the Muslim convert also attempted to persuade their youngest daughter, then 16, to adopt his faith and move to Birmingham with him.
But he failed following a bitter row with Ms Harvey and the girl remained in Kent with her mother.
Former Kent schoolboy Masood, born Adrian Elms but had several aliases in a long criminal career that included two prison sentences, met Ms Harvey in 1991 in a pub in Tunbridge Wells.
The couple separated nine years later when he was jailed for GBH. Their two daughters are now aged 19 and 24.
Speaking exclusive to MailOnline, a close friend of Ms Harvey said: "The elder daughter converted to Islam and is living in Birmingham.
"She wears a full face veil and I think had changed her name. It was her father who had helped convert her.
"He wanted the younger daughter to convert but Jane was against it and there was quite a family struggle.
'I know Jane was very upset and wanted her to stay at home and continue her studies."
The younger girl lives still lives with her mother, who is a director of a chemical company in Kent.
There is no suggestion that any of the family, including the elder daughter, is a radical Islamist or knew about her father's extreme beliefs or his plans.
The new details of mass murderer Masood's life illustrate his decline from respectable schoolboy to violent criminal and subsequent fanatical terrorist.
A friend of mother-of-two Ms Harvey, 48, was stunned to discover he was the attacker responsible for the mayhem and four deaths in Westminster.
Masood was shot dead by a close protection officer after killing three on Westminster Bridge and stabbing to death PC Keith Palmer at the entrance to Parliament.
He met successful businesswoman Ms Harvey in 1991 and they split in 2000 when he was jailed for GBH.
The friend said the tumultuous relationship was punctuated by Masood's violence towards others, who was then known by the name Adrian Elms.
"It was a very nasty time and the police spoke to Adrian on many occasions.
"I think Jane was very glad to get out of the relationship. We were all very happy for her."
Describing the attempt by fanatic Masood to convert his daughters, the source said he would fill her head with how good the religion was and tell her she would be better off if she converted. Jane is not at all religious.'
The friend said she last saw the eldest daughter when she came to visit her mother in hospital.
"She was covered head to toe and said she never went out of the house unless she was covered up."
"Jane was very upset when she converted but she was over 18 and there was not much she could do about it."
They added Ms Harvey first met Masood at a pub called The Roebuck, which according to the family friend, had a reputation for violence.
"I wasn't that close to him but would see him in there. The pub had a reputation for attracting the wrong sort of people. Then he was just known as Ade. The pub was very rough and it attracted a lot of football thugs, mostly from Chelsea.
"He was part of that group and it wasn't the nicest place to hang out.'
It is not thought Masood and the mother of his two daughters ever married. He went on to marry another woman in 2004, but the couple divorced and the woman lives with her new husband in Oldham.
Ms Harvey is the director of three companies, including a successful chemical company in the village of Bodiam, Kent.
She is also the director of property management company and a business that deals with catering.
The blonde is listed as a director of Aaron Chemicals with her father Peter also listed as a co-director.
Masood claimed on a CV that he was a sales director of the same company having worked for the family run business for 12 years.
But there is no proof the Westminster killer was with the company for that period or attained a senior position.
The company have refused to comment.
Neighbours of Ms Harvey's parents near Tonbridge, Kent, said they would often see her and the children - but never saw her partner.
"The grandkids would be round here with Jane quite often, but we never saw him. I think one of the children was visually impaired," said one neighbour.
The retired couple were not at home but appeared to have left their detached house in a hurry, leaving one of the doors of their garage ajar.
"This must have come as an awful shock to them. They are a very quiet couple but it is their grandchildren that I feel sorry for. They have done nothing wrong but they will always have a father who caused such horror and misery."
Questions remain over where Masood was radicalised.
Reports suggest it could have been during one of his stints behind bars, while others point to time allegedly spent working in Saudi Arabia, home to some of the most virulent Islamic extremism.
His family moved around during his youth.
His mother Janet Elms married Phillip Ajao in 1966 and the killer called himself Adrian Russell Ajao as one of his many identities over the years.
His criminal career dated back to 1983 and included convictions for assault, GBH and possession of an offensive weapon.
This led him to spend time behind bars at Lewes Prison in East Sussex, Wayland Prison in Norfolk and Ford open prison in West Sussex.
One of his jail terms was a two-year stint for a fight in 2000 outside the Crown and Thistle in Northiam, near Rye in East Sussex that left a man needing 20 stitches in his face.