Ronnie Biggs was sentenced to 30 years in jail
Great train robber Ronnie Biggs has "serious pneumonia" but is in a stable condition, his son Michael has said.
The 79-year-old was moved from Norwich Prison to the nearby Norfolk & Norwich University Hospital on Friday.
Michael Biggs, 34, said he hoped to visit his father but had been told by the prison authorities that he would not be allowed to see him.
The Prison Service said the families of inmates in hospital for short stays were discouraged from visiting them.
"He has got a serious case of pneumonia and he is being drip-fed antibiotics but is in a stable condition," said Mr Biggs, who lives in London.
"I have been told he is not allowed to have any visitors at all in the hospital.
"Apparently it is standard procedure that families are not allowed to visit for the first 48 hours if a prisoner is in hospital, for security reasons."
Mr Biggs urged the authorities to show "compassion" and allow him to see his father.
"I could understand it if he was still a hardened criminal - I could see then that there may be a security risk.
"But my dad is a very sick elderly man and I think they should show some compassion and allow his family to be with him."
He said he expected his father to be returned to prison on Tuesday.
A Prison Service spokeswoman there were no set rules about visiting prisoners in hospital.
"No prisoner is given special privileges," she added.
"The family of prisoners are discouraged from visiting them during short stays in outside hospital because of the potential for disruption to other patients and the management of security."
Juliet Lyon, director of the Prison Reform Trust, said: "It's time government thought again about how best to deal with thousands of elderly offenders, many of whom are chronically ill, disabled and demented.
"A compassionate country would develop secure accommodation for the elderly, or allow them home at last, rather than suffer the double punishment of growing old and dying in a bleak jail."
Biggs became infamous for his part in the so-called Great Train Robbery.
On 8 August 1963, a 15-strong gang including Biggs attacked the Royal Mail's Glasgow to London post office train, making off with a record £2.6m haul.
The train driver was severely injured and the gang's hideout, complete with fingerprint evidence, was soon uncovered.
Biggs escaped from Wandsworth Prison in a furniture van after serving only 15 months of a 30-year-sentence.
He used plastic surgery to change his appearance and moved to Spain, Australia and then Brazil.
On his return to the UK he was sent to Belmarsh Prison from where he was transferred to Norwich Prison, which caters for elderly inmates serving long sentences.