Ralf Schumacher (right) believes Michael is not enjoying F1 this year

The defending champion has not won a race this season and is enduring his longest barren run since 1993.

"I don't believe that Michael will be driving for very much longer," Ralf told German newspaper Sport Bild.

"He'll stay as long as he's having fun but is not having fun at the moment and I'm anxious to see what will happen."

Michael Schumacher, 36, has been offered an extension to his Ferrari contract, which runs until the end of 2006, but has not yet signed it.

He won 13 of the 18 races last season but is languishing in fifth place in this year's overall standings - 35 points behind leader Fernando Alonso of Renault.

When Michael leaves, a lot of good Ferrari people will leave at the same time Ralf Schumacher

"Ferrari have passed their zenith," said Ralf Schumacher, adding that the team's problems were caused by more than their under-performing Bridgestone tyres.

"They did a great job in past years but they're not doing it anymore.

"Mistakes have crept in other areas. It can be seen as a sign of weakness that Ferrari introduced their new car so late in the season.

"I'm not sure whether someone who has had such a successful time, as Ferrari has had, can still have the bite to fight back to the top."

Ralf, who drives for Toyota, predicted that Ferrari would undergo an upheaval when his brother retires.

"When Michael leaves then a lot of good people will leave at the same time," he said.

"After that it will be very difficult for Ferrari to get back to the top. I'm not sure that would even be possible."