The former world champion died in a high-speed crash during the San Marino Grand Prix in 1994.

The Williams' team boss, Frank Williams, his technical director, Patrick Head, and the former chief designer, Adrian Newey, had all been charged with manslaughter.

The lawyer for the Williams team, Peter Goodman, said his clients were obviously happy with the verdict.

"I would think they'll be very pleased that this part of the process is over and that it has gone successfully for them," he said.

The verdict was made in a makeshift courtroom in Imola, not far from the circuit where a statue now commemorates the Brazilian former triple world champion who crashed and died there.

Despite the ruling, the trial raised as many doubts as it did answers, and exactly why Senna lost his steering that day may now never be known.

The prosecution had alleged that a poor weld on Senna's steering column had snapped as the Brazilian entered the famous bend, the Tamburello, causing him to lose control of his car.

It also claimed that the way the track was maintained could also have contributed to the accident.

But the assertions were vigorously challenged by Williams as well as other drivers.

"There was no proof that it [steering column failure] caused the accident, it must have been something else," Senna's former Williams team mate, Damon Hill, said.

The track claim was dismissed by Imola officials and the Italian driver, Michele Alboreto, who said the asphalt surface had caused some problems but not enough to throw a car off-line.

The lawyers for Williams had indicated that Senna, a driver who rarely made mistakes, may have lost control while trying to avoid a piece of debris on the track. But this also has never been proved.

Last month, the prosecuting magistrate, Maurizio Passarini, made a dramatic about-turn when he asked for manslaughter charges against Williams and the three track officials to be dropped "for not having committed the offence."

Under Italian law, the prosecution have an automatic right to appeal against the verdict, but that now seems unlikely.