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Mercedes is at the centre of a growing controversy ahead of the Monaco Grand Prix after it emerged the team had conducted a private 1000 kilometre tyre test at Barcelona after the last race.

The Brackley-based outfit has struggled to work out why its race form is no match for its qualifying efforts this season, having now secured four consecutive pole positions but struggled with tyre wear in the races.

AUTOSPORT can reveal that rival outfits have been left deeply unhappy that Mercedes was given a golden chance to boost its understanding of the issue with a private test at Barcelona last week. Due to weather interruptions, it extended over three days.

Although in-season testing is banned, it is understood that Mercedes was requested by Pirelli to help work through the issues that the Italian tyre manufacturer has experienced in the last few races.

According to sources at Mercedes, the test was allowed because Pirelli can call upon one outfit for a private test to help it with products if circumstances dictate.

Mercedes is adamant that its running at Barcelona was totally within the regulations, but high-level sources have revealed that other teams are not convinced and the matter has now been taken up with the FIA.

Team principal Ross Brawn said that his outfit had worked tirelessly on the tyre issue since Barcelona - but did not make reference to the test when asked about what the team had done since the Spanish GP.

"I think when we got back from Barcelona we had some pretty long and deep meetings and decided a course of action," he explained.

"There were a lot of things going on already, so I don't want to create the impression that we got back and all of a sudden we started work. There was a lot of work going on already.

"I think some of the work was refocused in areas that we thought were maybe more relevant and in the last couple of weeks, Bob [Bell] has been directing all the engineering team to make sure that they focus on the areas that we felt were most important.

"We have come here with changes, and it is almost impossible at Monaco to judge those changes but we believe that they are relevant and give another snap shot of that at Montreal.

"But it won't be until we get to Silverstone that I think we can judge where we stand and there are more changes that we are trying - and trying to establish those changes in the best possible way with the limitations of going to races."