Edwin Van Der Sar challenges rival Manchester United keeper Ben Foster

Edwin Van Der Sar has warned Ben Foster he must earn the right to replace him as Manchester United No.1 and book his place at next summer's World Cup.

Foster is desperate to play more regularly for United next season to give himself the best possible chance of forcing his way into Fabio Capello's England squad for South Africa.

Although this season is likely to be 38-yearold Van der Sar's last, he has told Foster he is in no mood to give up his No.1 status.

"The manager has gone public and explained that he's planning to give Ben more games," said the Dutchman.

"I think that's normal because you have to look to the future. But I still want to play as many games as I can next season.

"As far as making a decision on my future, I'm going to see how the season goes.

"Several factors will determine my decision. Am I still enjoying it? Do the club still want me? Those kind of things."

Foster, whose career has been blighted by a succession of frustrating injuries, has long been touted by Fergie as a future England No.1, but the United boss admitted out here that the 26-year-old must stay fit to have any chance of fulfilling his potential.

"Ben has a challenge to take Edwin's positon because he's one of the greatest goalkeepers of all time," said Fergie.

"We just hope and pray Ben gets a long run without an injury."

Despite his injury history, Fergie clearly has faith in Foster - he handed him a new four-year contract worth £40,000 a week earlier this month.

United arrive in Germany today following the completion of their four-game tour of the Far East.

Fergie's men are competing in the Audi Cup in Munich, playing two games on Wednesday and Thursday before flying back to Manchester to continue their pre-season build-up.