Former Wales, Arsenal and Celtic striker John Hartson has testicular cancer which has spread to his brain, it has been confirmed.
The 34-year-old, capped by his country 51 times, was diagnosed on Sunday.
His career started at Luton and his £2.5m transfer to Arsenal in 1995 was then a record fee for a teenage player.
His consultant in Swansea said further test results would "establish a full picture" of his diagnosis and he faced "many months" of treatment.
A course of radiotherapy and chemotherapy would start at the city's Singleton Hospital "as soon as possible".
Dr Gianfilippo Bertelli, consultant medical oncologist for Abertawe Bro Morgannwg University NHS Trust, said Hartson had attended last Friday with symptoms consistent with testicular cancer.
"A number of clinical tests were put in place and these have confirmed the presence of testicular cancer," he said.
"Unfortunately, it has also been confirmed that this cancer has spread to Mr Hartson's brain.
"We are awaiting the results of further tests to establish a full picture of Mr Hartson's diagnosis."
Dr Bertelli added: "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."
His family released a statement saying: "All the family, and John's friends, are doing all we can to support him and praying that he will make a full recovery.
"John is getting excellent care from the oncologists at the hospital as he begins his treatment.
"John is a family man and we appreciate all the messages of support we have already received."
His former club Celtic issued a statement saying everyone fully supported "John and his family at this extremely difficult time".
This 2008 footage shows footballer John Hartson speaking about his retirement
Hartson moved to Glasgow in 2001 after being signed by then Celtic manager Martin O'Neill for £6m and spent five years at Parkhead, scoring 89 goals.
He was voted player of the year by his fellow professionals and the Scottish Football Writers Association.
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.
He began his senior career at Luton in 1992 as a 17-year-old and the player's bustling style soon caught the eye of bigger clubs.
Arsenal paid £2.5m for Hartson in January 1995.
A £3.3m move to their London rivals West Ham followed two years later, although that stay was marred by an altercation with team-mate Eyal Berkovic.
In 1999 Wimbledon paid £7.5m for Hartson, while spells at Coventry, Celtic, West Bromwich Albion and Norwich - on loan - followed before he announced his retirement as a player in February 2008.
When the 6ft 1in target man retired he said his biggest regret was not having played for his beloved Swansea City, the local club he had supported since a boy.
Since his retirement as a player, Hartson has proved himself an articulate and knowledgeable football commentator for media outlets, including the BBC.
Hartson seemed destined to move into football management and has been linked with vacant posts at the likes of Falkirk, Swansea and East Stirlingshire.