The FIA has said that all 11 Formula 1 teams should have been given the opportunity to take part in a recent tyre test.
Mercedes conducted three days of running following the Spanish Grand Prix, with drivers Lewis Hamilton and Nico Rosberg piloting a W04, something which Red Bull and Ferrari has protested, believing the test to contravene sporting regulations.
Mercedes say they had approval from the FIA but the sport's governing body say they cannot be sure conditions of the test were met by Mercedes.
"At the beginning of May, the FIA was asked by Pirelli if it was possible for it to carry out some tyre development testing with a team, using a current car," read an FIA statement issued after the Monaco Grand Prix.
"Within the contract Pirelli has with the FIA as a single supplier, there is provision for them to carry out up to 1000km of testing with any team - provided every team is offered the opportunity to do so"
"Pirelli and Mercedes-AMG were advised by the FIA that such a development test could be possible if carried out by Pirelli, as opposed to the team that would provide the car and driver, and that such tests would be conditional upon every team being given the same opportunity to test in order to ensure full sporting equity."
"Following this communication, the FIA received no further information about a possible test from Pirelli or from Mercedes-AMG. Furthermore, the FIA received no confirmation that all the teams had been given the opportunity to take part in this test."
After a post-race protest in Monte Carlo, the stewards forwarded their findings to the FIA, who could put the issue to their International tribunal.
Mercedes team principal Ross Brawn insists no rules have been broken.
"Pirelli has been asking teams to help them out for 12 months and people haven't been supporting them, there are lots of communications between Pirelli and teams asking them to do 1000km for them, and we obviously had an issue in Bahrain with Lewis which we were quite anxious about – and we made the effort to help them. Nobody else seems to have done that."
