The Bethnal Green schoolgirl tried to recruit younger women to join the jihadist group, according to reports.
According to the Sunday Telegraph, the 19-year-old played an active role in the organisation’s reign of terror and was allowed to carry a Kalashnikov rifle, earning herself a reputation as a strict “enforcer” of laws, such as dress code.
Allegations also emerged today that Ms Begum had been witnessed preparing suicide vests for would-be bombers.
The Mail on Sunday said Prime Minister Theresa May and Home Secretary Sajid Javid had been briefed that she sewed vests onto bombers so that they could not be removed without them going off.
The information was said to have been gleaned by allied spy agencies, believed to be the CIA and Dutch Military Intelligence - from other Western ISIS converts. It is not known if Begum was a willing participant.
Ms Begum has previously insisted she was never involved in the terror group’s brutality and said she spent her time simply as a housewife, after fleeing London when she was just 15.
She said her interests were on caring for her children, who have since died, and her Dutch national husband, Yago Riedijk.
Sources told the Telegraph the teen arrived to Syria after crossing the border in Turkey in February, 2015. From there, she was taken to the central city of Raqqa.
Ms Begum is said to have graduated to a level where she was a member of an all-female “police squad”.
In this role, it is claimed Ms Begum carried a Kalashnikov rifle and had a reputation for being strict on other women who she accused of behaving in a “non-Islamic way”.
In this role, Ms Begum is said to have received anything between “£500 and £1,500 a month”.
The source also alleged Ms Begum was active in recruiting other women to join ISIS from across Europe, including texting a teen from Austria in 2015.
She allegedly told one girl: “Don’t believe any of the bad things you hear about Dawla [the State], it’s fake. You have everything you want here and we can help find you a good-looking husband.”
Ms Begum had her British citizenship revoked and has since been stuck in a displacement camp for months after being detained when leaving ISIS territory.
The teenager arrived heavily pregnant at al-Hol camp in February, and gave birth shortly after. But her newborn son, named Jarrah, died from a lung infection last month. It was her third child to have died during her time living in the caliphate.
Ms Begum’s family are proceeding with a judicial review of Home Secretary Mr Javid’s decision to remove her citizenship.
Her father Ahmed Ali said last night: "If she has done anything wrong, she should be brought to England and be punished."
Of the latest claims, the Begum's family lawyer demanded the Home Office make any evidence held against her public.
Tasnime Akunjee said: "On behalf of the family, we would like to see this evidence, given that it has been leaked, and have it tested in proceedings in due course rather than through hearsay."