One of the biggest days of the Formula One season - the Monaco Grand Prix - has been overshadowed by controversy around a Mercedes tyre test.
The team's two drivers - Nico Rosberg and Lewis Hamilton - took the front row of the grid in qualifying yesterday. Soon after, other teams discovered that they had run an apparently secret 600 mile test on Pirelli tyres.
Testing during the season is banned, but Pirelli has insisted the test was legal under the regulations.
Red Bull's team principal, Christian Horner, has protested to race stewards and said: "What annoys me is that it was so underhand. And it makes a mockery of the rules.
"First of all we need to deal with it through the proper channels, and that's what we'll look at doing.
"There is confusion between what is contractually permissible and what a team is allowed to do in the sporting regulations. It's a situation we need clarification on, and our position is we are going to request that. I can understand Pirelli wanting to test the tyres, they've obviously got issues with them.
"What's disappointing is that it has not been done in not a transparent manner, that a three-day test has taken place with a current car running on tyres that are going to be used at the next grand prix. Irrelevant of what you call it, that's testing."
Paul Hembery, Pirelli’s motorsport director, said: “It is completely regular in that we are allowed to do a 1000 kilometre (600 mile) tyre test with any team,” Hembery said. “In the World Rally Championship contract it is exactly the same. We can do it with a representative car. We have done it before with another team.”
Mercedes team principal Ross Brawn said: "We were approached by Pirelli to conduct a tyre test because they were very concerned about some of the problems they were having recently and they didn't feel the car they had been using was representative.
"In their agreement with the FIA, there is a provision to ask any team to do a 1000km test. We ran that past the FIA to make sure they were happy – and they were."
Pirelli's tyres made the news during the Spanish Grand Prix two weeks ago, when most drivers had to take four pitstops.
Mercedes struggled particularly badly in that race, with Hamilton dropping to twelfth from second on the grid, complaining that he couldn't handle the tyres.