England skipper Rio Ferdinand suffered an injury worry during the squad's first training session in South Africa on Friday.
The Manchester United central defender sustained a knee injury during the final couple of minutes and has gone to hospital to undergo a scan.
England manager Fabio Capello is uncertain at this stage of the severity of the injury. But he confirmed: "Rio picked up a knee problem in training. It happened near the end. He has gone to hospital for a scan and we will have to see how things are."
Ferdinand had been optimistic of enjoying an injury-free World Cup after a season dogged by problems at United.
He was able to feature in barely a third of their Barclays Premier League campaign but had trained with no setbacks during England's two-week camp in Austria and also came through the friendly internationals with Mexico and Japan unscathed.
Goalkeeper David James also missed training but Capello is less concerned with that situation and insisted the 39-year-old had "only a slight knee problem".