* Mercedes in secret tyre test with Pirelli
* Red Bull and Ferrari protest
* Governing FIA to decide course of action (Updates with stewards' action, fresh quotes, FIA statement)
By Alan Baldwin
MONACO, May 26 (Reuters) - Red Bull and Ferrari protested against Formula One rivals Mercedes on Sunday after accusing the Monaco Grand Prix winners of breaking the rules by carrying out a secret tyre test with Pirelli last week.
Stewards said in a statement, after Germany's Nico Rosberg had celebrated his victory, that they would send a report to the governing FIA, who could bring the matter before the sport's International Tribunal.
Mercedes - whose other driver is 2008 world champion Lewis Hamilton - could face a heavy sanction, including possible exclusion from the championship, if found to have acted illegally. The race results were, however, declared official.
Red Bull team principal Christian Horner said he had found out about the test in Barcelona only on Saturday after the details leaked out in a meeting of the Grand Prix Drivers Association.
He told reporters, amazed that the test had been kept secret in a sport full of prying eyes, that Mercedes had behaved in an "underhand way" and suggested they had gained an unfair advantage.
"We feel testing in Barcelona in a current car with current tyres is in breach of the regulations," he said. "It's effectively a contravention of the rules that affect the championship."
The regulations ban in-season testing but Pirelli said their contract allowed them to do a number of 1,000-km private tests with a 'representative' car and the three days in Spain were therefore legal.
"The testing rules are very clear in the sporting regulations," said Horner. "When you enter the championship at the beginning of the year you sign up to those regulations. And in our opinion Mercedes by doing that test have not complied.
"Whenever you run these cars you are learning about reliability, about the mechanical side of the car and how these tyres act and behave. For Mercedes to claim they didn't benefit from that test would be difficult to believe."
FIA UNAWARE
Mercedes GP non-executive chairman Niki Lauda said the team had sought and obtained permission from the FIA.
"It is very simple. We were asked by Pirelli, we asked the FIA: 'Are we allowed to do the test?' The FIA confirmed it and said 'yes' and so therefore we did the test. We think other teams have been asked too," he said.
"Mercedes did nothing wrong - they asked the right people for permission. We asked the FIA, the FIA checked it legally and advised us we could do it so we can't do any better."
Lauda's version of events was challenged by the FIA, who said in a statement hours after the race had ended that Pirelli and Mercedes had not told them about the test beyond an initial query in early May.
The governing body said Pirelli had asked whether they would be allowed to test with a team using a current car.
"Pirelli and Mercedes were advised by the FIA...that such tests would be conditional upon every team being given the same opportunity to test in order to ensure full sporting equity," the statement said.
"Following this communication, the FIA received no further information about a possible test from Pirelli or from Mercedes. Furthermore, the FIA received no confirmation that all teams had been given an opportunity to take part in this test."
Pirelli motorsport head Paul Hembery, already under fire from Red Bull over the current tyres, told reporters before the race that Mercedes could "absolutely not" have gained any competitive advantage.
The team had struggled with tyre wear during races, despite having racked up three successive poles before Monaco.
Horner, whose drivers finished second and third in the race, said the action by Mercedes meant a testing agreement between the teams "might as well be declared null and void now".
"That obviously hasn't been respected either," he said.
Hembery said the rain-affected test in Barcelona had been 90 percent aimed at the 2014 tyres and "Mercedes haven't got a clue what on earth we were testing in reality."
Asked why the other teams had not been told in advance, Hembery suggested that would have been counter-productive.
"You know in Formula One that when you start talking about something six months could pass before you found a solution," he said. "There's also another point of looking at it... that things take far too long. In reality sometimes you just have to get on and do it." (Editing by John O'Brien, Clare Fallon and John Mehaffey)