Tony McCann was serving a minimum of 21 years behind bars over the killing of 33-year-old Lou "Luke" Hodge's in 2008.
Nicola, who played Cindy Beale's sister Gillian Williams, is believed to have stuck by McCann, who is the father of her twin daughters Tilly and Dora, 21, while he was in prison.
A family source said he had been diagnosed with liver and kidney cancer around six months ago, The Sun reports.
He died of cancer at the high-security prison HMP Gartree in Leicestershire on Monday, October 19.
The Prison Service said: “HMP Gartree prisoner Tony McCann died on October 19 and the Prisons and Probation Ombudsman has been informed.”
He had accused him of stealing a £20 mobile phone to sell for drugs and of bullying his friends.
He fled to Nicola's home and demanded a lift to a friend's house.
A day earlier McCann, a drug addict with convictions for violence since he was 15, had told a policeman "something has to be done" about Mr Hodge.
Nicola, 52, appeared sporadically on EastEnders from 1998 to 2014.
At McCann's sentencing at The Old Bailey in 2008, Nicola wept as a judge said McCann must serve at least 21 years behind bars.