Major title rivals Red Bull Racing and Ferrari beginning a protest against Mercedes with the Monaco race stewards.
Following the complain Formula 1 governing body FIA says that 2013 F1 chassis at the latest Mercedes tyre test should have give an opportunity to every team to take part.
"What's wrong is that a team, in an underhand way, consciously tested tyres that were designed for this year's championship," Horner said.
"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."
