Embedded in the nose cone of two of its cars was a button-sized diamond, as part of a promotion for the forthcoming Hollywood film Ocean's 12.

But Jaguar's afternoon lost its shine when the Austrian driver Christian Klien crashed his car and the £140,000 gem vanished.

Amid accusations of a publicity stunt, investigators were searching for the missing jewel last night.

Klien, 20, making his debut on the calendar's trickiest circuit, ploughed his green Jaguar into the barriers at the Mirabeau corner on the opening lap.

The car, with the flawless diamond the size of a shirt button set into a steel ring, was lifted off the track where it sat for nearly two hours. When it arrived at the Jaguar garage a mechanic discovered that its most valuable component was missing.

Klien said: "That will be the most expensive drive I'll ever take around Monte Carlo."

A diamond had been attached to Klien's car and that of his teammate Mark Webber as publicity for the sequel to the Hollywood hit Ocean's 11, starring George Clooney.

The film features a diamond heist and Jaguar bosses had set up the promotion with Steinmetz, an Israeli diamond dealer.

Jaguar said: "Christian lost his steering and hit the barriers, forcing him to retire. His car returned to the garage minus his flawless diamond which has yet to be found."

Last night there was speculation a fan may have pocketed the stone.

Nav Sidhu, a Jaguar spokesman, said: "Someone here has walked away with more than a motor racing souvenir."