It has emerged in Monaco that Mercedes took part in a 1000km private test with Pirelli after the Spanish Grand Prix, leading its rivals to argue it has gained an unfair advantage.

Pirelli has been complaining about a lack of testing in a contemporary car for some time and wanted to complete the test to gain more mileage on its tyres. Under the regulations in-season testing is banned but Mercedes says it checked with the FIA before conducting the test and believes it was legal.

Red Bull is among the teams questioning the issue as it believes the in-season test ban makes the test illegal. Article 22.4 of the Sporting Regulations forbids testing between a ten-day period before the start of the season and December 31 of the same year, with the two exceptions being a young-driver test and four straight-line tests.

"I can understand Pirelli wanting to test the tyres, as they have obviously got issues with the tyres," Christian Horner told Autosport. "What is disappointing is that it has been done in a not transparent manner.

"A three-day test has taken place with a current car on tyres that are going to be used in the next grand prix. Irrelevant of what you call it, that is testing."

However, Pirelli is allowed to conduct testing under its agreement with Formula One and has previously used a 2010 Renault. With much higher levels of downforce this year, the Italian tyre company has struggled to simulate race conditions and has experienced excessive degradation and delaminations.

The test with Mercedes, which took place at the Circuit de Catalunya with both Lewis Hamilton and Nico Rosberg at the wheel of the 2013 car, was completed at Pirelli's request and will have given the Italian tyre company crucial mileage ahead of a plan to bring in a new construction of rear tyres at the Canadian Grand Prix. But it claims Mercedes had no idea which tyres it was testing and a lot of the work was focused on 2014.