Pirelli's appearance at the FIA International Tribunal is related to questions over whether not inviting other teams to the Mercedes test was "prejudicial" to the best interests of Formula 1.
As proceedings got underway at the FIA's headquarters in Paris, the governing body's legal representative Mark Howard laid out the reasons behind Mercedes and Pirelli being summoned.
While there seemed little doubt over why Mercedes, which used its 2013 car in the secret test, was under scrutiny, the reasons for Pirelli's summons had been initially unclear. Howard explained that doubts about whether other teams had been invited to take part in the test – or even had notification about it taking place so they could send observers – needed to be examined.
"None of the other 2013 competitors were invited to participate in the test or observe," he said. "None of the other 2013 competitors was aware that the test was to take place."
Howard said that F1 race director Charlie Whiting had received some informal correspondence about the theoretical possibility of a 2013 car being tested, but was never informed that the Barcelona test was going to take place.
He added: "Without the knowledge, consent and participation of other competitors, Mercedes and Pirelli may have engaged in activity that was prejudicial to the competition."
Howard cited that such actions could be a breach of article 151c of the sporting code, which states that a breach is: "Any fraudulent conduct or any act prejudicial to the interests of any competition or to the interests of motorsport generally."
It was clarified that Pirelli is not being accused of breaching its contract with F1.
Howard also stated that the FIA believes Mercedes did gain an advantage from the test, and questioned whether the team had had access to test data.
"Running the car for three days at least informed Mercedes that they did not need to change the cars," he stated. "That in itself would be valuable information.
"It is not beyond the bounds of possibility that some defect would have become apparent in the three days track testing, and it cannot seriously be suggested that Mercedes would not seek to remedy that defect. We would suggest that it is difficult to say Mercedes obtained no benefit from the test."