David James says he will be fit for England's World Cup campaign in South Africa next year, despite missing the friendly against Brazil on Saturday. Fabio Capello had warned that he would not consider the Portsmouth goalkeeper for the finals if he continued to be dogged by a knee problem.

However, James told the Guardian tonight: "I'm confident that I will be right for the World Cup. There is no 'knife-edge' situation regarding my fitness."

James has been struggling with "irritation" of the knee following surgery on his posterior cruciate ligament at the end of last season. With Capello's strict selection criteria emphasising full fitness as essential, there has been speculation over whether James has slipped down the pecking order, having missed the last England squad as well as the game against Belarus last month. But James is adamant that his knee will not prove a barrier to his England chances. "My knee won't stop me being fit for 2010. I've got four months now before the next England game and I will be fit and available for selection."

If picked for the next friendly, the 39-year-old would earn his 50th cap for England and be in a good position to boost his claim on the No1 spot in South Africa. He is England's most experienced keeper and Capello would probably be reluctant to travel to South Africa without him.

James believes that is not an option. "It's not a serious injury, it's just a niggle that we're managing. I know the England manager is concerned about training after games, he has a very strict policy and, if you can't do his training before a game, he won't pick you. That's fair enough."

He said the perception he is unable to train after games is not quite true. "I don't miss any training sessions at Portsmouth. These days I'm in every Monday and Tuesday after a match on the weekend. For the last three years at Portsmouth I've just done gym work on a Monday.

"In the old days, under Harry Redknapp, he was happy for me to do my gym work at home because of the long commute to the training ground from Devon – and the team were only doing 45 minutes on the pitch anyway – Harry's view was so long as I was performing in matches he was happy. These days I do come into training on a Monday but we still mostly do gym work so there is really very little difference."

"I could do the England sessions but the priority for me had to be to rest and recovery. Having last week off was a good opportunity for me because I hadn't had any time off – other than missing the Man City and England games back in August, my knee hasn't prevented me from preparing for or playing in any of my Portsmouth games since."

James believes that an over eagerness to return to training in pre-season has been the cause of his prolonged rehabilitation. "I wasn't able to get it right in pre-season after rushing back to training too quickly following the operation. The doctors felt I recovered well from the surgery, I thought I was back to normal but in my first training session I was doing 60-yard kicks which I hadn't done in months and it caused irritation."

The keeper, who turns 40 next summer, says he would not risk either his own career, or the success of his country if he was not up to the job. "I wouldn't risk jeopardising myself or England if I wasn't right. This last week off has been a positive for me, and I'm looking forward to getting back on track now."