All eleven F1 teams were last year advised of the opportunity to help Pirelli with its tyre programme, but were expressly told that any testing could not take place once the race season had begun.

A Formula One Teams Association (FOTA) communiqu?, subsequently leaked to Britain's Daily Telegraph newspaper, amongst others, in the wake of the Mercedes test row [ see separate story], further underlines the mire that the Monaco Grand Prix winner - and potentially Pirelli - could find itself when the FIA sits to ponder protests lodged by rivals including Red Bull and Ferrari.

Both Mercedes and Pirelli spent raceday in Monaco claiming that the three-day session, conducted in Barcelona immediately after the Spanish Grand Prix, was above board, despite Red Bull's Christian Horner deeming it 'underhand' [ see separate story] amid accusations that both parties had conspired to keep it a secret.

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While Mercedes' Ross Brawn has attempted to torpedo the latter allegation, pointing out that his rivals should have been aware that Mercedes wasn't exactly packing its equipment away following the Barcelona race, the latest revelation casts a longer shadow over Nico Rosberg's Monaco victory. Mercedes is already thought to have broken the rules by providing a 2013 car - and its two race drivers - for the test, and the timing of the session, which Pirelli insists was primarily to test tyres for next season, now appears to be a further transgression by both parties.

"Pirelli is entitled under the terms of their agreement with the FIA to offer teams 1000km of tyre testing, subject to each team being treated equally," the leaked letter confirms, "However, there are no provisions within the sporting regulations for such testing to take place in-season."

The missive also underlines the point already made that the test could only be staged with the unanimous support of all the teams, which should have been offered the same opportunity to run, or at least be present during the session. Pirelli insists that all teams were contacted about the opportunity to help with the development programme, but that only a few, including, responded.

Despite both Mercedes and Pirelli claiming that the test had been approved by the FIA - indeed, Brawn says that his team contacted the governing body for clearance after the invitation to run in Barcelona - a statement from Paris has since stated that there had been no such consent, and that the green light would only have been given had all eleven teams been offered the opportunity to participate [ see separate story].

The FIA is currently considering its option, but it looks increasingly likely that Mercedes will face sanctions for its part in the test. Pirelli's fate remains unclear, meanwhile, but it has to be remembered that the tyre supplier is currently in the last year of its existing contract and waiting to sign an extension with the teams [ see separate story].