Confirmation that the FIA has summoned Mercedes to face an international tribunal following its Barcelona test with Pirelli has produced a couple of twists to an already convoluted tale.

Both the team and tyre supplier insist that they were given approval to conduct the 1000km session at the Circuit de Catalunya with Mercedes' 2013 car, despite such an action being contrary to the F1 rulebook, which prohibits contemporary machinery from being employed lest it provide an unfair advantage at a time when in-season testing is expressly outlawed.

Mercedes' defence is now rumoured to include an email from FIA race director and safety delegate Charlie Whiting which, it is alleged, granted permission for the W04 to be used as Pirelli attempted to get to grips with both a spate of tyre failures and preparations for 2014.

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The speculation, if true, could put the governing body - which cleared Ferrari of a similar testing misdemeanour because it used a 2011 car in Bahrain - in the tricky position of having to investigate the actions of one of its own. Whiting apparently revealed that the test had taken place during a pre-Monaco GPDA meeting.

However, Mercedes' claims that the test was above board took another blow to their credibility when it emerged that Nico Rosberg and Lewis Hamilton wore plain helmets during the three-day session, disguising their involvement.

Asked about the apparent deception during Thursday's press conference ahead of the Canadian Grand Prix, the German apologised and insisted that he 'did not want to comment'.