Daniel Sturridge, the Chelsea forward, is undergoing emergency treatment for a suspected viral meningitis.
The 22-year-old was taken ill over the weekend after repeated vomiting and severe aching, before he contacted the club's medical staff. He then had tests at St Mary's Hospital in Paddington.
Blues manager Roberto di Matteo is scheduled to hold a press conference on Wednesday, when it is likely he will reveal more on the England international's health situation.
Sturridge scored 13 goals in 43 appearances for the Stamford Bridge club last season as they won an FA Cup and Champions League double.
The former Manchester City player was selected by Stuart Pearce to represent Team GB at the London 2012 Olympics on Monday, however his participation is now in doubt.
Team GB play their first match on 26 July - two weeks on Thursday - against Senegal. Pearce has until 25 July to call up a replacement, with Huddersfield Town's prolific striker Jordan Rhodes the probable replacement.
Pearce's team will also face the United Arab Emirates on 29 July and Uruguay on 1 August.