Ralf Schumacher will miss the next two Grands Prix because injuries he sustained in a crash at the US Grand Prix in June have not healed.
Schumacher's spinal injuries have not yet sufficiently healed
The German had hoped to make a return in Hungary on 15 August but has been ordered by doctors to sit the races out while two broken vertebrae recover.
A Williams statement said Schumacher's "convalescence continues well" but said it was "prudent" to wait for a return.
Brazilian Antonio Pizzonia will replace him for the second race running.
Schumacher's manager Willi Weber told Bild newspaper in Germany: "The two vertebrae are not yet as hardened as is normally the case.
"Another accident like the one in Indianapolis and Ralf could be made a paraplegic. Nobody could accept that."Schumacher said: "I feel good, but it's no doubt better for me to listen to the doctors."
Williams said there car would feature changes in Hungary, with a revised nose design.
A team statement said it was more "traditional" in appearance than the stub nose that has marked out the FW26 since the beginning of the season.