Ronnie Biggs has served two years of a 30-year jail term

Biggs, 73, had been taken from Belmarsh Prison to the Queen Elizabeth II Hospital in south London.

He was left "very weak and very frail" by the illness and it was initially thought he had suffered a heart attack.

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

His bid to appeal against his 30-year jail term was dismissed in October.

His son, Michael, 29, had been at his bedside since he was taken to hospital on Tuesday night.

"From what the doctor has told me, they don't think he had a heart attack last night but believe it is a severe case of pneumonia.

"They think this may have made his heart beat in a different way."

A spokeswoman for the hospital said Biggs had been discharged at around 1545 GMT and taken back to Belmarsh.

Biggs has already suffered two minor heart attacks since returning to the UK, and had three strokes and two epileptic seizures.

The robber was caught and jailed for his part in the infamous �2.6m heist on a Glasgow-to-London mail night train in 1963.

But he escaped from Wandsworth Prison in 1965 and fled abroad, spending most of his time in Rio de Janeiro.

His son Michael says keeping him in Belmarsh high-security prison when he is so unwell is a waste of taxpayers' money.

"My father came back to close a chapter in his life. He is an Englishman who wanted to come home.

"It would be nice if the present government was to show some compassion and at least take him out of a maximum security jail."