Schumacher tested a 2007 Ferrari at the team's Mugello test track on 31 July
Red Bull and Toro Rosso have joined Williams in blocking Michael Schumacher's bid to test Ferrari's 2009 car ahead of his Formula 1 return.
Ferrari had written to the other nine teams and the sport's governing body, the FIA, asking for the German to be granted a day at the wheel of the F60.
But three teams insisted in-season circuit testing must remain strictly prohibited, as FIA rules dictate.
Schumacher will replace Felipe Massa at the European Grand Prix on 23 August.
The 40-year-old Schumacher, who is standing in after Massa was seriously injured at the Hungarian Grand Prix, began his preparations by driving a 2007 Ferrari at the team's Mugello test track on Friday.
But in-season testing with this season's cars is banned under current regulations.
Williams sees no distinction between Alguersuari's situation and Schumacher's and feels any deviation from the rule would create a precedent for the future
Five members of the Formula 1Teams' Association (Fota) - McLaren, Renault, Toyota, BMW Sauber and Brawn GP - were, given the circumstances, prepared to overlook that and give Schumacher and his Italian team the green light.
Ferrari have said the German, who won a record 91 grands prix in a 15-year career, will stand in for Massa until the Brazilian is fit enough to make a return after suffering a fractured skull in an accident during qualifying for the Hungarian Grand Prix.
Williams say seven-time world champion Schumacher should not be given special dispensation, citing the recent arrival of 19-year-old rookie Jaime Alguersuari into F1 when he replaced Sebastien Bourdais for Toro Rosso.
"While we welcome Michael Schumacher back to F1, the fact is any form of in-season circuit testing is strictly prohibited, a regulation clearly laid out by the FIA and adhered to by all of the teams," said Williams in a statement.
"It was for this reason Alguersuari, who drove an F1 car for the first time in Hungary, did not have the opportunity to familiarise himself with the Toro Rosso before he made his race debut.
"Williams sees no distinction between Alguersuari's situation and Schumacher's and feels any deviation from the rule would create a precedent for the future."
Williams also pointed out that Ferrari chose Schumacher over current test drivers Luca Badoer and Marc Gene, who have both tested the team's 2009 model.
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"In a similar situation, Williams would unhesitatingly use its current test driver," added Williams.
"For the sake of consistency and fairness, therefore, we oppose Ferrari's proposal to test ahead of the European Grand Prix."
But Mercedes motorsport boss Norbert Haug told autosport.com that his company would be happy to allow Michael Schumacher to test the 2009 Ferrari before his comeback, and that he hoped rival teams take the same stance.
"Whatever we can do to support him we will do," he said.
"I just think it is fair. This is an exception. If Michael had said he would come back under the condition that he could test, everybody would have said yes.
"His team-mate has thousands of kilometres in this car, and he has nothing. As we are fair players, why should he not get some testing? I would be very open to that."
F1 supremo Bernie Ecclestone said he understood the teams' thinking in opposing allowing Schumacher practice time.
"What they're saying is it opens the door for the future. People can say 'it was ok for Michael'," said Ecclestone.
"The young Spanish guy came in for his first race (Alguersuari), he didn't do any testing, so they'd say 'why didn't he?'
"If we don't mind, there should always be someone who wants to test, and we're trying to stop testing."
Schumacher, who retired at the end of the 2006 season, had not driven an F1 car since April 2008 before he tested at Mugello on Friday.
"A great feeling to be back in an F1 car," he said afterwards. "After a few laps, I was able to drive constant times and I am quite happy with the time I did.
"Now, we will have to see how my body and my muscles will react to that day in the next (few) days."
On Thursday he spent a day at the Ferrari factory in Maranello, Italy, familiarising himself with the controls on the 2009 car's steering wheel in the team's simulator.
The car was fitted with slick tyres, which were reintroduced for this season and which Schumacher has not raced on since 1997.
Just before Massa flew home to Sao Paulo on Monday, after leaving the hospital that had treated him for the fractured skull he suffered on 25 July, the Brazilian said he hopes to recover quickly so he can return to racing.
Massa was hit on the head by a spring that had come off Rubens Barrichello's Brawn while he was travelling at more than 160mph during qualifying at the Hungaroring.
F1's governing body, the FIA, has launched an investigation into the accident.