Biggs marked his 75th birthday on Sunday in Belmarsh Prison

They told BBC News Online they were confident a judicial review of his sentence at Belmarsh Prison, south-east London, would succeed.

Biggs has suffered two strokes and his solicitors say he has facial paralysis which means he cannot speak or eat.

He was 75 on Sunday - 41 years to the day since the notorious �2.6m robbery.

Papers seeking permission to apply for a judicial review were lodged with the High Court on Monday.

The application will be considered by a judge before being referred for a court hearing at a later date.

He poses no risk to the public Venetia Bombas Solicitor Son calls for father's release

She said Biggs has to be fed liquids through a tube powered by an electric pump.

"He is physically very frail. He is sick and sad. He suffers from depression at being in a high security prison," she said.

But mentally he was "fully coherent" and a "very intelligent man."

She said they had a new medical report on Biggs's condition which has deteriorated recently and she said this was first time the Manchester-based solicitors had represented him.

"It is very unjust to keep him in a high security prison. He poses no risk to the public," she said.

She estimated it costs about �55,000 a year to keep him in prison with medical care.

"He has no ability to speak, his handwriting is shaky and he communicates by using a sheet of A4 paper," she added.

He does not interact with other prisoners but has never had any problems with other inmates or officers.

Voluntary return

Previous attempts to transfer Biggs to a lower security prison or be released on compassionate grounds have failed.

Biggs was caught and jailed for his part in the infamous heist on a Glasgow-to-London night mail train in 1963.

He escaped from Wandsworth Prison in 1965 and fled abroad, becoming a Brazilian citizen and spending most of his time in Rio de Janeiro.

He has been held in Belmarsh since he returned voluntarily to the UK in May 2001 - 36 years after escaping from jail and fleeing abroad.

Belmarsh is a centre of medical excellence where we can provide 24-hour care Prison Service Profile: Ronnie Biggs

But Biggs' solicitors are challenging the refusal to transfer or release Biggs as violation of both Prison Service orders and the Human Rights Act.

A Prison Office spokesman said Belmarsh was a centre of medical excellence where Biggs could be provided with 24-hour care.

"At a lower category prison there would not be this specialist round-the-clock care", the spokesman added.

Editor of the Prisons Handbook, Mark Leech, said: "It is very unlikely this bid will be successful.