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Ferrari and Red Bull have launched an official protest against rivals Mercedes, accusing them of doing illegal testing.

Formula 1 tyre supplier Pirelli used Mercedes to do a tyre test between the Spanish and Monaco Grands Prix. In-season testing in F1 is forbidden.

Pirelli says it has a contract with the FIA allowing limited testing.

But Red Bull and Ferrari say this must only be with a car at least two years old. Mercedes used their 2013 car.

Rival teams were not informed about the test, which took place at Barcelona's Circuit de Catalunya after the track had hosted the Spanish Grand Prix two weeks ago.

Mercedes ran for three days using race drivers Lewis Hamilton and Nico Rosberg.

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.

"They have run a current car with current drivers and current tyres. That's a clear breach of the sporting regulations and we've protested because we want clarity.

"It's not right for a current team to test a current car on tyres we're going to be racing on here or in two weeks in Montreal.

"I can understand Pirelli wanting to test the tyres," he added. They've obviously got issues with the tyres.

"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."

Red Bull's protest is under article 22.4 of the sporting regulations, which dictates that no on-track testing may take place during a season other than a designated three-day 'young driver test' or straight-line aerodynamic testing.

The only exception is if a car of at least two years old is used.

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."

Any number of potential penalties are open to the stewards if they decide Mercedes have broken the rules, from exclusion from the world championship, down to a fine.

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.