John Hartson has undergone emergency neurosurgery to relieve pressure on his brain, the hospital treating him said today.
It was revealed earlier this week that the former Celtic, West Ham and Arsenal striker had been diagnosed with testicular cancer, which had spread to his brain.
A statement from Morriston Hospital in Swansea said cancer had now also been diagnosed in his lungs.
The statement from Abertawe Bro Morgannwg University NHS Trust said: "John Hartson was transferred from Singleton Hospital to Morriston Hospital yesterday evening (Tuesday), where he received emergency neurosurgery to relieve pressure on his brain.
"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. Unfortunately, cancer has now also been diagnosed in his lungs.
"He will resume radiotherapy and chemotherapy as soon as possible."
Hartson's family said in a statement: "His partner Sarah, mum, dad, three children, brother, two sisters and very close friends are all continuing to support John in any way we can.
"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.
"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.
"Finally, we would like to ask that John's privacy, and that of our family, is respected as John continues his treatment in hospital."
The 34-year-old attended Swansea's Singleton Hospital on Friday with symptoms of testicular cancer.
Tests were then carried out to establish a full picture of his diagnosis.
Dr Gianfilippo Bertelli, consultant medical oncologist at ABM University NHS Trust, said earlier this week that a course of radiotherapy and chemotherapy would be started as soon as possible and that he could foresee many months of treatment.
"Mr Hartson will continue under the care of the specialist consultant oncologists as we aim for a positive outcome from this treatment," he said
"We realise that Mr Hartson is a much-loved figure and there are many fans and well-wishers who will be keen to hear news of his progress.
"However, while treatment is ongoing, we would ask that the privacy of Mr Hartson, and all the patients who attend our hospital, is respected."
A Celtic spokesman said: "He has served the club, and the game of football in general, with distinction over many years.
"Clearly, everyone at the club fully supports John and his family at this extremely difficult time."
Hartson was signed to the Glasgow club by Martin O'Neill in a £6 million deal in 2001 and was a popular figure with fans during the five years he spent at Parkhead.