RED BULL team principal Christian Horner has accused Mercedes of being "underhand" in conducting a secret tyre test for which they face the threat of serious sanctions.
Red Bull were joined by Ferrari in launching a protest ahead of the Monaco Grand Prix after inadvertently discovering a three-day test took place in Barcelona from May 15-17.
It seems news of the test emerged on Friday during a Grand Prix Drivers' Association briefing, with the suggestion Nico Rosberg – who won yesterday – or Lewis Hamilton let it slip out.
With the sporting regulations banning in-season testing, Horner is in no doubt Mercedes – in conjunction with tyre supplier Pirelli – are in contravention of the rules.
"What's wrong is that a team, in an underhand way, consciously tested tyres that are designed for this year's championship," said Horner.
"As far as we're concerned the regulations are black and white.
"It's a team's responsibility to comply with the regulations, so the issue isn't so much with Pirelli.
"When a team runs around for three days in Barcelona on a tyre that is going to be used at the next grand prix when in-season testing isn't allowed, it is, in our opinion, an unfair advantage."
The matter was addressed post-race by the stewards who are to submit a report to motor sport's world governing body the FIA "who may bring the matter before the International Tribunal".
Should that be the case, and if found guilty, Mercedes can expect to receive a fine, points penalty, race ban or even be excluded from the championship.
Mercedes team principal Ross Brawn is adamant the test was not secret, seemingly laying some of the blame at Pirelli's door.
"It was up to Pirelli to spread the information. It wasn't up to us, it was their test," said Brawn.