A House of Lords debate was suspended on Monday night, after a Labour peer collapsed in the chamber.
Lord Brennan was taken to hospital by ambulance
Lord Brennan, 65, had just been speaking in the debate on the Human Fertilisation and Embryology Bill, when he was taken ill.
Health minister Lord Darzi, a leading surgeon, was among those who gave Lord Brennan a heart massage.
He was treated by ambulance staff in the chamber before being carried out on a stretcher and taken to hospital.
A spokesman for Guy's and St Thomas' Hospital said: "Lord Brennan was admitted to the accident and emergency department at 8.43pm and his condition is stable."
Lord Brennan, a QC, was chairman of the Bar Council in 1999. He is the Home Office's independent assessor on miscarriages of justice.
The second reading debate on the Human Fertilisation and Embryology Bill was later adjourned to Wednesday.