Back in the early days of Lewis Hamilton's Mercedes tenure, a scandal enveloped the team over a 'secret test' it had conducted with Pirelli.
F1 rules at the time dictated that bar the official young driver test, testing was expressively banned in-season.
But Nico Rosberg's victory in the 2013 Monaco Grand Prix was overshadowed by the news of Mercedes conducting a three-day, 1000km test with its 2013 car, with the German and Hamilton as drivers, on behalf of Pirelli after the Spanish GP.
This is that story.
Mercedes and Pirelli tyre test
The 2013 Mercedes, the W04, had a problem: it had a ferocious appetite for Pirelli tyres.
The rear tyres, in particular, degraded alarmingly quickly, to such an extent that during the Spanish GP, Hamilton, who had qualified in second place, was lapped, finishing 12th for his worst result until the 2021 Azerbaijan GP nearly eight years later.
Pole-sitter Rosberg finished sixth, more than a minute down on winner Fernando Alonso as Mercedes tried desperately to understand why its car was so tyre-hungry.
Tyre supplier Pirelli had a contract with the FIA to allow it to do some limited running and approached Mercedes to do so to test new tyres set for introduction later in the 2013 season.
Both Pirelli and Mercedes then approached the FIA's Charlie Whiting for approval for the test with the W04. Although Whiting was not in a position to formally green-light the running, his word was considered impeccable. It is believed he signalled that it could take place if the other teams were afforded the same opportunity.
Pirelli had already asked the other teams in general terms if they would run when asked, and both Pirelli and Mercedes deemed it to be sufficient to run the test in the days after the Spanish GP.
Both Hamilton and Rosberg wore blacked-out helmets instead of their traditional designs, as rival teams Red Bull and Ferrari caught wind of the test on Saturday evening in Monaco.
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Mercedes and Pirelli in hot water
Predictably, Red Bull's Christian Horner cried foul and immediately declared that the test lacked transparency.
"They have run a current car with current drivers and current tyres," he said at the time. "That's a clear breach of the sporting regulations and we've protested because we want clarity.
"It's not right for a current team to test a current car on tyres we're going to be racing on here [in Monaco] or in two weeks in Montreal.
"I can understand Pirelli wanting to test the tyres. They've obviously got issues with the tyres.
"What's disappointing is it's been done in not a transparent manner. A three-day test has taken place with a current car running on tyres that are going to be used in the next grand prix and irrelevant of what you call it, that's testing."
Mercedes and Pirelli were subsequently hauled in front of an FIA tribunal, which found that the governing body had given "qualified approval" for the test.
The tribunal, however, found that Mercedes had broken Article 22.4 of the sporting regulations in conducting a test in-season with a current car, and that the FIA's approval, handed out in "good faith" by Whiting, did not and could not override Article 22.4.
Whilst it was found that neither Pirelli nor Mercedes had acted in bad faith or gained any "unfair sporting advantage", there was the possibility of Mercedes earning "an unfair sporting advantage".
It was also found that as permission had been sought for the test, Mercedes had "no reason to believe that approval had not been given".
As punishment, Mercedes was banned from taking part in the 2013 young driver test, with both it and Pirelli reprimanded for their actions.
In-season F1 testing was re-introduced in 2014 after specific grands prix, with Pirelli now rotating in-season testing through teams with current cars.
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