The 34-year-old had initially found out that he was battling testicular cancer but doctors have since told him that the disease has since spread to his brain.
Hartson came to West Ham in February 1997 and forged a formidable partnership with Paul Kitson that ultimately saved the Hammers from relegation that season.
The following season he went on to score 24 goals for the Hammers in the season they finished 8th in the Premiership.
Hartson finished his Hammers carear with 33 goals in 73 games before joining Wimbledon for a fee of £7.5 million in 1999.
All our best wishes go to Hartson and his family as he battles this horrible illness.