Mercedes face an investigation by Formula 1's governing body the FIA after being accused of taking part in an illegal tyre test.

Red Bull and Ferrari lodged an official protest at the Monaco Grand Prix after discovering tyre supplier Pirelli used Mercedes to do a three-day test.

Formula 1 tyre test row explained

In-season testing in F1 is forbidden. Pirelli says it has a contract with the FIA allowing limited testing.

The Monaco GP stewards are to prepare a report for the FIA.

A statement external-link said the governing body "may bring the matter before the International Tribunal".

It added that under the terms of its contract with the tyre manufacturer it had allowed Pirelli to carry out some tyre development testing with a team, using a current car, "provided every team is offered the opportunity to do so".

It adds: "Following this communication, the FIA received no further information about a possible test from Pirelli or from Mercedes-AMG. Furthermore, the FIA received no confirmation that all teams had been given an opportunity to take part in this test."

The International Tribunal, effectively for these purposes the FIA's court, can impose any number of penalties, ranging from exclusion from the world championship to a fine.

Ferrari team principal Stefano Domenicali said: "As it is to do with the sporting regulations, you might expect a sporting penalty, but because it is not really clear what could be the effect on the race weekend, it is maybe bigger than that.

"I honestly don't know what should be the solution. Because there is no precedent, I have no idea what should happen."

Red Bull and Ferrari have accused Mercedes of breaking article 22.4 of the sporting regulations, which forbids in-season testing other than for a single three-day young driver test or for very limited straight-line aerodynamic tests.

quote 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

Pirelli say they are allowed to ask teams to do up to 1,000km of testing, which was as much as Mercedes did over three days at the Circuit de Catalunya in the immediate aftermath of the Spanish Grand Prix, the race before Monaco.

Rival teams were not informed about the test, which not only involved Mercedes' 2013 car but also race drivers Nico Rosberg and Lewis Hamilton.

Rival teams are angry because extra testing could give Mercedes a technical advantage. Mercedes say they sought permission from the FIA, which approved the test.

Mercedes have been struggling with heavy tyre usage this season.

The tyres used were development designs for next season and a new tyre Pirelli wants to introduce at the Canadian Grand Prix, the next event after Monaco.

Pirelli is aiming to introduce rear tyres that are less vulnerable to delamination following a series of failures. But the plan is in dispute - Ferrari, Lotus and Force India have yet to approve it.

Red Bull team principal Christian Horner said he had only learnt about the test on Saturday night.

"I can understand Pirelli wanting to test the tyres," he said.

Formula 1: Monaco Grand Prix highlights

"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."

He added: "We need to deal with it through the proper channels so that's what we'll look at doing."

A Ferrari spokesman added: "Pirelli can offer to the teams the chance to do 1,000km of testing for tyre development and safety. But the fundamental aspect is the year of the car because if you use a current car it should be allegedly a breach of article 22.

"We want a clarification on this because if it is not against article 22 we would be interested in doing this."

Mercedes' test is the second tyre test conducted by Pirelli this season. The first was done by Ferrari between the Bahrain and Spanish Grands Prix but involved a 2011 car.

Insiders say Mercedes' tyre problems this year made it logical for Pirelli to use their car in assessing new tyres aimed at preventing failures.