Motor sport's governing body called a hearing Sunday after Red Bull and Ferrari launched an official protest against Mercedes alleging that the team breached rules by conducting in-season tyre testing with manufacturer Pirelli following the Spanish Grand Prix two weeks ago.

The FIA said in a statement that the three Formula One teams and Pirelli would answer questions following Sunday's Monaco GP, which was won from pole position by Mercedes driver Nico Rosberg ahead of Red Bull pair Sebastian Vettel and Mark Webber.

"We feel it's not in line with the rules so that's why we've protested before the race," Red Bull team principal Christian Horner said. "We just want clarity."

Mercedes has been lightning fast in qualifying — securing a fourth straight pole position at the Monaco GP — but had struggled with durability during races and Rosberg's win was its first of the season.

Rosberg would not be drawn on the issue, saying only "you'll have to ask Pirelli about this issue. I don't want to comment."

Although Lotus did not lodge a protest, team principal Eric Boullier was also critical of the tyre testing and said it fell outside of regulations.

"At the end of the day it's a breach of the sporting code," Boullier said, adding that he found out about the tests on Saturday night. "If they did it, I think it is maybe because they think they could get an advantage."

In agreement with Mercedes, Pirelli conducted tyre testing following the Spanish GP, which is not permitted under the sport's rules. However, Pirelli insists its contract allows it to conduct private tests with individual teams under special circumstances.

F1's governing body has yet to comment but Pirelli's head of motorsport, Paul Hembery, said there was no wrongdoing.

"It's completely regular in that we are allowed to do a 1,000-kilometer tyre test with any team," he said prior to the start of Sunday's race. "In the World Rally Championship contract it's exactly the same. We can do it with a representative car. We've done it before with another team, and we've asked another team to do some work as well."

Hembery said the testing was to look at possibilities to develop next year's tyres — providing Pirelli is still the official manufacturer — and in no way gave Mercedes any advantage in Monaco.

"Absolutely not, because it's no relevance to what's happening here," he said. "Mercedes haven't a clue what on earth we were testing in reality."

Niki Lauda, Mercedes GP's non-executive chairman, added on SkySports television that the FIA gave permission to conduct the testing.

"We asked the FIA 'Are we allowed to do this?' The FIA confirmed it and said 'yes', so therefore we did the test," Lauda said. "These tests can always come in cases of emergency, so Mercedes did absolutely nothing wrong."

Horner is unhappy that the testing took place at all.

"First of all we need to deal with it through the proper channels, and that's what we'll look at doing," Horner said. "It's a situation we need clarification on, and our position is we are going to request that clarification."

Boullier added even if there was an agreement between the FIA and Pirelli, it was still unfair on other teams.

"Whatever permission is given, it should be allowed to everybody, or at least to make it aware to everybody and not testing on your own somewhere," Boullier said, adding that Pirelli had not offered Lotus a chance to test.

Pirelli has been under pressure in recent weeks because of the fast-degrading nature of its tyres — with nearly 80 pit stops at the Spanish GP.

Red Bull, led by Horner and championship leader Vettel, has been one of the fiercest critics of the Pirelli tyres, and the manufacturer still does not have the backing of all teams — or any word from the FIA — over a new contract for next season. Pirelli responded to the criticism over the weakness of its tyres by recently pledging to make modifications at next month's Canadian GP.

The fact that Mercedes has been struggling for race pace, and that Pirelli has been trying to improve the resistance of its tyres, seemingly gives extra meaning to the private testing.

"Irrelevant of what you call it, that's testing," Horner said. "They've both cars on the front row of the grid, so it's not hurt."

However, Mercedes has been fastest in qualifying for four straight races so it is debatable whether the testing will have made any difference to gaining pole in Monaco.

Red Bull had won the past three Monaco GPs.

Although last year's winner Webber was sportsmanlike in praising Rosberg, he still feels Mercedes has questions to answer.

"I don't think it probably had a huge bearing on today's results, That car would always perform pretty well around here," the Australian said. "But you can't unlearn what went on at the test. So we need to see how it came about and what the rules are to see why Mercedes thought it would be OK."