Coulthard (left) does not think Schumacher is in decline

Seven-time world champion Schumacher has been reduced to an also-ran this season as Ferrari have struggled.

"Whether he is in decline, I don't know. I can only observe, and I would be surprised if he was in terms of his potential performance," said Coulthard.

"All you can do is compare Michael to his team-mate (Rubens Barrichello), and he is pretty much in the same place."

Ferrari dominated Formula One for five years from 2000-2004 but thanks to a variety of reasons, including running on uncompetitive Bridgestone tyres, they have been overtaken by the likes of Renault, McLaren and even BAR this season.

With an inferior package compared to his rivals, Schumacher has not been able to offer a challenge to the likes of championship leader Fernando Alonso or second-placed Kimi Raikkonen, and Coulthard feels that is a shame for the sport.

Whoever wins the championship this year can't say they've beaten Michael Schumacher because the Ferrari is not in the same formula David Coulthard

"What everybody would like to see is three or four competitive teams battling for the championship, and Michael being part of that," said Coulthard.

"Whoever wins the championship this year, Fernando or Kimi, they will not have actually beaten Michael.

"Yes, they will have finished in front of him, and they will have won the title, but they can't say they've beaten Michael Schumacher because the Ferrari is not in the same formula.

"Right now you've the Michelin formula and the Bridgestone formula. That's the reality of the sport because tyres are so influential.

"For a more pure world championship, you need to have one manufacturer. That would take away a very big variable.

"I've raced in F1 when there's been one tyre manufacturer, and obviously now when there is competition.

"The great thing with competition is being part of it, developing something that makes the car go quicker. There's nothing better.

"But then the downside is if you end up with the wrong tyre - more so than with engine or chassis - you can't win. As simple as that.

"As best as I can tell, all indication is their problem is tyres, and that's affecting the whole package."