The 80 year old was admitted to Barnet General Hospital in London on Saturday (29May10) after complaining of agonising discomfort in his chest.
He is now said to be in a "stable" condition, but his son, Michael Biggs, is worried about his father's failing health.
He explains, "He had some very severe chest pains... He's also got skin cancer which has been flaring up. It's a real worry, because his health really isn't improving. He's an old man and has been ill for a while - but hopefully he'll pull through... He's conscious but he's in a lot of pain."
The famous thief, who was part of the gang behind 1963's Great Train Robbery, was granted a pardon on his 80th birthday in August (09) and freed from prison on "compassionate grounds" due to ill health, but has been in "good spirits" after the birth of a granddaughter.
Michael adds, "My daughter was born in the last few weeks. He's been in good spirits celebrating that."
Biggs escaped from prison in 1964 and spent more than 30 years on the run before returning to the U.K. in 2001 when he was arrested and sent back to jail.