Alexander Wurz' heavy crash, on the final day of testing at Paul Ricard, has once again raised concerns over safety.

An initial investigation suggests that the cause of the crash was tyre failure, though both McLaren and Michelin are still investigating.

The French manufacturer has confirmed that the two rear tyres on Wurz' car have been returned to its factory at Clermont Ferrand for further analysis. However, it's not thought that the findings of the investigation will be known until late next week.

Wurz' failure comes less than a week after Michael Schumacher's double tyre failure at Barcelona, and will add to concern within the pitlane that the new tyre rule, whereby tyres have to last the entire weekend, is dangerous.

Following last weekend's failure, which caused the German's retirement from the Spanish Grand Prix, Ferrari President blamed the Japanese manufacturer, which responded by stating that it would not compromise safety.

The accident also comes at a time when David Coulthard is calling for increased safety at F1 tests.

Following the Austrian's crash on Friday morning, the test was moved to a different configuration for the afternoon, which did not use the high-speed straights.

In 1986, the highly rated Elio de Angelis was killed in a mysterious test crash at the French track.