Thomas Reay, aged 39, is behind bars at HMP Doncaster for armed robberies in Newcastle in which a gang stole thousands of pounds.
He was sentenced to an indeterminate sentence for public protection (IPP) and ordered to serve a minimum of six years but has served 11 so far.
Under IPP legislation, prisoners are only released once the Parole Board no longer deems them a risk.
A petition has been launched calling for Reay’s release and the prisoner has been on hunger strike for nearly two weeks to highlight his case.
His family has also written to Boris Johnson.
A concerned relative said: “I emailed Boris Johnson a week ago but nothing back.
“Anything to do with terrorists, they’re sharp on it. Here we have a young man, no danger to anyone, being left to die.
“How dare they, he’s paid for what he did.”
In his latest message to his followers on Facebook page set up to provide updates on his case, Reay said: “I think the prison system and HMP Doncaster are going to push this to the very end. Let’s hope if it comes to that my death isn’t in vain and others get the help they need. Release IPP prisoners.”
IPPs were introduced in 2003 and designed for serious sexual and violent offenders but were used more widely than had been anticipated and were scrapped in 2012.