Christian Horner has said he feels that Mercedes' tyre test with Pirelli was carried out in an 'underhand way' and was a clear breach of F1's Sporting Regulations.

Red Bull issued a protest against Mercedes - along with Ferrari - shortly before the start of the Monaco Grand Prix after news surfaced of the three-day test at Catalunya, where the 2013 Mercedes was used to help Pirelli with development work as it seeks to overcome the issues it has faced so far this season.

Mercedes and Pirelli both insisted that they had received permission from the FIA to conduct the test, although the governing body has since issued a statement revealing that certain conditions were attached - including that the test should be 'carried out by Pirelli, as opposed to the team that would provide the car and driver' and that the test was dependent on 'every team being given the same opportunity to test in order to ensure full sporting equity'.

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Speaking in Monte Carlo, Horner said he felt that the test was a clear breach of the regulations, and said the way in which it had been carried out was 'underhand' and that the rules would now need to be revisited on the back of what had happened.

"What's wrong is that a team, in an underhand way, consciously tested tyres that were designed for this year's championship," he was quoted by ESPN. "We view that the testing rules are very clear in the Sporting Regulations and when you enter the championship at the start of the year you sign up to those regulations.

"In our opinion, in doing that test, Mercedes hasn't complied and therefore we protested before the race because we wanted it to be clear that it wasn't a question of what result they have and that at the first opportunity we had to put in a protest; we put it in because we want clarity.

"We talk an awful lot about saving costs and we spent three hours on Friday talking about in-season testing and trying to find a solution and it turns out one team has already done a huge amount of it. There also is a testing agreement, that may as well be declared null and void now, which is a contract between the teams that isn't governed by the FIA and that obviously hasn't been respected either."

Red Bull advisor Helmut Marko also hit out at Mercedes over the test, arguing - unlike the two Red Bull drivers - that the team had benefitted from the extra session on track.

"We are very unhappy," he told the Daily Telegraph. "When we test for three days, we go a second faster - that's what Adrian Newey says. It definitely helped them - you can see that they had no tyre problems today. That's no accident.

"There are sporting regulations that cannot be overridden by a civil agreement between Pirelli and the FIA. The sporting regulations state clearly when and how you can test. We are seeking clarification of how to proceed and that the competitive advantage of Mercedes is compensated in some way."