Michael Schumacher's bid to test Ferrari's current car has been blocked by Williams.
Ferrari last week wrote to the other nine Formula One teams and the FIA requesting Schumacher be granted a day at the wheel of the F60 ahead of his return to racing at the European Grand Prix in Valencia on August 23 in the absence of the injured Felipe Massa.
Under current regulations in-season testing is banned, but with 40-year-old Schumacher barely turning a wheel in anger since retiring just under three years ago, Ferrari were hoping for leniency.
Out of courtesy, the other seven members of the Formula One Teams' Association - McLaren, Renault, Toyota, BMW Sauber, Brawn GP, Red Bull Racing and Toro Rosso - gave Schumacher and Ferrari the green light.
Williams, however, see no reason why seven-time world champion Schumacher should be given special dispensation, citing the recent arrival of 19-year-old rookie Jaime Alguersuari into F1.
"While we welcome Michael Schumacher back to Formula One, 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 further noted Ferrari chose Schumacher over current test drivers Luca Badoer and Marc Gene, who are both familiar with the team's 2009 model.
"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."