Allegations by FIA president Max Mosley that an unnamed Formula One team was guilty of cheating last year have been questioned by Williams technical director Patrick Head, who also cast doubt on the merits of an FIA plan to introduce changes to the sport's rules at the British Grand Prix in two weeks time.

Head was responding to statements made by Mosley in Imola in which the FIA president said that the FIA had become aware during the winter that an unnamed F1 team had breached the sport's regulations governing the use of electronic systems, which could be interpreted as similar to the traction control systems outlawed by the sport at the end of the 1993 season.

In response to this knowledge Mosley said systems, such as the pit-lane speed limiters (currently legal and fitted to all F1 cars), would become illegal as they were deemed to be possibly contributing to a form of traction control.

"Until a certain point last year, I believe we were policing the electronics very satisfactorily. But then it came to our notice that something was going on which we believed was wholly unacceptable," said Mosley "As you know, we download all the team's computer programs, and we keep records, so we are able to conduct a continuous audit.

"This demonstrated that something had slipped through the net. I am not about to name names or to identify the team involved, but this incident showed that we could no longer make the assumption - one which we had always made in the past - that major companies would not be involved in actions that were blatantly against the rules."

Asked directly if he believed a team had run an illegal car during last season Mr Mosley said he "believed this to be the case". The FIA president then described the measures to be taken at Silverstone to eradicate the possibility of cheating.

"In time for Silverstone, what we have done is to examine all the cars and, figuratively speaking, cut a number of `wires', to make it more difficult for the electronics to be exploited in ways that we regard as inappropriate," said Mosley. However, Head, voicing the concerns of many in the pit-lane, said the introduction of any changes to the rules would have to be carefully considered.

"There is a mechanism, which is fully understood and agreed by the FIA and the teams, by which rules can be changed and those mechanisms have to be followed and I'm hoping that those mechanisms will be followed between now and Silverstone," he said. "You can't just change rules unless it's on the grounds of safety, without following agreed and contracted procedures."

Head's concerns were echoed by Prost team principal Alain Prost who added that any knee-jerk changes to the regulations could have unforeseen repercussions.

"I can't disagree with it, but for me the timing is a problem. It could be more difficult to achieve than people think and could introduce some safety problems. I think it's a little bit too soon."

Earlier Mosley had also suggested that FIA investigations into possible cheating was ongoing but if it came to light that a particular team had broken the rules then retroactive punishment could be meted out. Speculation as to the identity of the team being investigated is likely to be rife in the coming weeks, although Mosley insisted that the team under scrutiny had "nothing to do with the outcome of the world championship".

The developing row over the Silverstone changes overshadowed events on the track, where the drivers busied themselves with free practice for tomorrow's San Marino Grand Prix. The day was dominated by Ferrari with Michael Schumacher fastest in both morning and afternoon sessions.

Chief rivals McLaren had a patchy day, with David Coulthard third at the end of the afternoon session, while Mika Hakkinen, who had complained to Italian media that he was unhappy with the car his team had given him, finished sixth, behind the twin Jordans of Heinz Harald Frentzen (fourth) and Jarno Trulli (fifth).

"We had a very good session and I'm very happy with how the day went," said Frentzen. "We had no problems and were able to do lots of work on finding the best set-up.

"The car felt good right from the first outing this morning and I think we're in good shape for the weekend."