Ronnie Biggs is staying in overnight due to his deteriorating condition after undergoing an emergency blood transfusion.
The Great Train robber is being treated at the Queen Elizabeth Hospital in Greenwich, south east London.
It is thought he may have suffered internal bleeding.
Further tests are being carried out.
Biggs, whose 72nd birthday was last Wednesday, was transferred to hospital from top-security Belmarsh Prison in south east London.
The robber, who was on the run for 35 years before returning to justice from exile in Brazil earlier this year, has been in declining health and is being fed by a tube.
A hospital spokesman said Biggs was stable and had been brought in for "investigations".
Biggs's spokeswoman said: "It is too early to say what is wrong with him but he is undergoing tests and has had a transfusion.
"Michael (his son) is obviously extremely concerned."