Hartson scored 89 goals during his five seasons with Celtic
Former Wales, Arsenal and Celtic striker John Hartson is in a critical condition after emergency surgery on his brain following a cancer diagnosis.
At the weekend, 34-year-old Hartson was told he had testicular cancer.
The disease spread to his brain and on Wednesday, doctors confirmed it was also present in his lungs.
The father-of-three had undergone surgery at Morriston Hospital in Swansea, after being transferred from the city's Singleton Hospital.
A hospital statement read: "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."
After being diagnosed with cancer on Sunday, Hartson was admitted to Singleton Hospital where further tests were carried out.
Since retiring from a glittering career in football in February 2008, the striker has worked as a television pundit.
Relatives issued a statement on Wednesday saying that all close family and friends "are 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," it went on.
John Hartson has been working as a football pundit since retiring last year
"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."
The family also requested that the star's privacy, and that of his family, is respected as he continues his hospital treatment.
Officials at Arsenal have said the club is "deeply saddened" by news of his cancer.
The former centre forward joined Arsenal as a teenager from Luton in January 1995 and, in two seasons at Highbury, made 62 appearances, scoring 16 goals.
The player left Arsenal for West Ham United in February 1997 and continued his career with the Hammers, Wimbledon, Coventry City, Celtic, West Bromwich Albion and Norwich City.
"Everyone at Arsenal Football Club would like to take this opportunity to wish John, together with his family and friends, all the very best in the challenging times that lie ahead. Our thoughts are with you," the statement added.
During his international career, he scored 14 goals before stepping down in February 2006 to concentrate on his club football.
Hartson made his Wales debut in March 1995 in an away match against Bulgaria, often leading the line as a lone striker, until his last game in November 2005 against Cyprus.