Biggs was seriously ill in hospital before his release
Justice Secretary Jack Straw has been accused of allowing public opinion to cloud his judgement over the release of Great Train Robber Ronnie Biggs.
Chairman of the Parole Board Sir David Latham said there was no "rational reason" not to release Biggs earlier.
In July, Mr Straw rejected the panel's release recommendation on the grounds Biggs was "utterly unrepentant".
The government says Biggs's health had deteriorated before he was freed on compassionate grounds on 8 August.
Biggs is severely ill with pneumonia and before his release from hospital in Norfolk was reportedly unable to walk, read, write or speak and could not eat or drink.
Mr Straw has denied his U-turn was linked to the Scottish government's release of Lockerbie bomber Abdelbaset Ali al-Megrahi - also for health reasons - 12 days later.
Sir David, a High Court judge, said Mr Straw might initially have had a "gut feeling" that people would not like the release of a man who had not admitted a "really awful" crime.
"I think it was a genuine feeling on his part on the way the public might feel about it but that doesn't necessarily mean it was a rational decision," he said.
Had Biggs's lawyers challenged the ruling by judicial review, Sir David said, they would probably have been successful.
He said the public trust in release decisions could be undermined because of the "political element" currently involved.
A Ministry of Justice spokesman said Mr Straw had refused parole "principally because Mr Biggs had shown no remorse for his crimes, nor respect for the punishments given to him, and because the Parole Board found his propensity to breach trust a very significant factor".
"In the case for compassionate grounds, the justice secretary had to consider the medical evidence against well-established criteria. The medical evidence clearly showed that Mr Biggs is very ill and that his condition had deteriorated."
The "overriding priority" in parole decisions was public protection, he added.
Biggs, originally from Lambeth, south London, was among a 15-strong gang which attacked the Glasgow to London mail train at Ledburn, Buckinghamshire, in August 1963, making off with £2.6m.
The train's driver, Jack Mills, suffered head injuries during the robbery and never returned to work, dying in 1970.
Biggs was given a 30-year sentence, but escaped after 15 months and went on the run for more than 30 years, living in Australia and Brazil. He returned to the UK voluntarily in 2001 in search of medical treatment.
Sir David's comments came as the Parole Board published proposals to bolster its independence from government by becoming part of the Courts Service, rather than the Ministry of Justice.
It is also recommending that ministers lose the power to veto the transfer of prisoners from closed to open conditions - often the first step towards an offender's release.
Sir David also revealed the Board was facing a backlog of 2,000 cases, with some decisions 18 months overdue.