Mercedes didn't get permission to participate in the test in which they are accused of breaking Formula 1 rules, according to governing body the FIA.
Both Mercedes and Pirelli are facing FIA tribunal with the investigation currently on-going.
The investigation is looking into the three-day, 1,000km tyre test last month and FIA counsel Mark Howard said Mercedes didn't follow the correct procedure to give them permission.
Howard said that any permission was on condition that Mercedes informed other teams of the test, which they failed to do.
"There was no attempt whatsoever by Mercedes to involve the other teams in order to ensure that no perception of an advantage was obtained," Howard told BBC Sport.
But Mercedes counsel Paul Harris believes that his team haven't broken the rules because they did not conduct the test, Pirelli did.
"This was not a test undertaken by Mercedes. They are critical words in text of Article 22 [of the sporting regulations] - 'undertaken by'," he said.
He added: "The Pirelli test was not a test undertaken by Mercedes. It is irrefutable it is a test undertaken by Pirelli."
Team principal Ross Brawn admits that Mercedes may have gained some knowledge from the test but he insists they had no competitive advantage because they were unaware of the tyres they were testing.
"We didn't know what the tyres were; we didn't know what the detail objectives were of what Pirelli were doing," he said.
A guilty verdict for Mercedes or Pirelli could have some serious consequences. Pirelli is considering suing the FIA if they are hit with a heavy penalty. While Mercedes said they may have to consider their F1 future if the company's integrity is damaged.
The trial continues.
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