Bellfield, 55, is serving two whole life orders for killing 13-year-old Milly Dowler, Marsha McDonnell and Amelie Delagrange, as well as the attempted murder of Kate Sheedy.
He threatened legal action against attempts to stop the prison wedding after he became engaged to a female visitor last year.
He reportedly got down on one knee to propose in front of wardens at high-security HMP Frankland in County Durham.
Bellfield has won a bid to be granted up to £30,000 in legal aid after his lawyers cited the European Convention on Human Rights and the 1983 Marriage Act, according to the Sun.
The paper also reports that he claimed to have been the victim of discrimination after officers banned him from wearing an engagement ring.
A Ministry of Justice spokesman said: “Under current laws, there are no legal routes to block this marriage and we recognise the pain and anger this outcome will bring to his victims’ families.
“It is what has driven our plans to stop prisoners on whole life orders from marrying in prison through our new Victims and Prisoners Bill – ensuring this never happens again.”
The government unveiled plans in March to overhaul the parole system, which included aiming to ban criminals serving whole life orders from marrying behind bars.
Who is Levi Bellfield?
He is a former bouncer and wheel clamper from London who has 11 children with three women. By 2002, Bellfield had nine convictions and had spent nearly a year in prison before he was 20.
In 2002, he abducted and killed 13-year-old Millie Dowler when she was on her way home from school in Surrey. In 2003, he killed 19-year-old Marsha Louise McDonnell by hitting her over the head with a blunt instrument after she got off a bus near her home in Hampton.
He tried to kill 18-year-old Kate Sheedy in 2004 by hitting her with his car, but she survived her injuries. A few months later, Bellfield killed 22-year-old Amelie Delagrange in Twickenham.
He began a whole-life prison term for the murders of Marsha McDonnell and Amelie Delegrange 14 years ago. Bellfield was then found guilty of Milly Dowler’s murder in 2011 and was sentenced to another whole-life term.
In 2016, Bellfield converted to Islam while behind bars, and wanted to be called Yusuf Rahim.
Bellfield applied to get married while serving two whole-life sentences for murder.
He is reportedly engaged to a woman who began writing to him two years ago before becoming a regular visitor. She is said to be blonde and in her 40s but her identity has not been revealed.
The convicted killer is said to have gotten down on one knee to propose to the woman in front of prison staff, as reported by The Sun. Ministers objected to the engagement, while former Prime Minister Boris Johnson is said to be “sickened and appalled”.
However, it is still legal to marry in prison, with the permission of the governor.