Hartson is at Morriston Hospital in Swansea and has undergone surgery. A statement from the hospital supplied to the BBC reads: "He is currently being cared for by the critical care team at Morriston and is receiving round-the-clock care aiming to stabilise his condition.
"He will resume radiotherapy and chemotherapy as soon as possible."
Today, his family issued their own statement which reads: "We have been overwhelmed by the support and goodwill from many thousands of football fans, players, clubs and sporting figures and we have drawn strength from this support.
"He is receiving outstanding care from all the medical and surgical staff and we would like to thank everyone at both Singleton and Morriston, as well as the excellent ambulance staff."