McLaren's David Coulthard has called for a re-think on rules that allow Formula One's bottom six teams to run a third car on Fridays at Grand Prix weekends.

The Scot says the system should be re-evaluated after each race to prevent teams like high-flying BAR from gaining a significant performance advantage.

"That rule shouldn't be fixed for the entire year," he told British weekly Motorsport News on Wednesday. "BAR has established itself as the second or third best team but it can still run a third car all year.

"We can't do much running on Fridays any more -- but we're sitting watching guys who are really competitive doing three or four times as many laps as us."

BAR are allowed to run a third car, driven by British test driver Anthony Davidson, after finishing last season fifth overall.

The Honda-powered team are third in the championship and have been the revelation of the season so far, with Briton Jenson Button taking three podiums in five races and pole position at the San Marino Grand Prix.

Struggling McLaren have scored only five points -- three fewer than BAR's Takuma Sato -- with last season's championship runner-up Kimi Raikkonen having just one to his credit.