Red Bull were joined by Ferrari in launching a protest ahead of the Monaco Grand Prix after inadvertently discovering a three-day test took place in Barcelona from May 15-17.
It seems news of the test emerged on Friday during a Grand Prix Drivers' Association briefing, with the suggestion Nico Rosberg - who won on Sunday - or Lewis Hamilton let slip.
With the sporting regulations banning in-season testing, Horner is in no doubt Mercedes - in conjunction with tyre supplier Pirelli - are in contravention of the rules.
"What's wrong is that a team, in an underhand way, consciously tested tyres that are designed for this year's championship," said Horner.
"As far as we're concerned the regulations are black and white, are very clear what you can and cannot do.
"It's a team's responsibility to comply with the regulations, so the issue isn't so much with Pirelli.
"It's more that the team has purposefully tested a current car, at a current circuit and with the current drivers which is in breach of the regulations.
"When a team runs around for three days in Barcelona on a tyre that is going to be used at the next grand prix when in-season testing isn't allowed is, in our opinion, an unfair advantage."
The matter was addressed post-race by the stewards who are to submit a report to motor sport's world governing body the FIA "who may bring the matter before the International Tribunal".
Should that be the case, and if found guilty, Mercedes can expect to either receive a substantial fine, points penalty, race ban, or worse case scenario, be excluded from the championship.
An FIA statement read: "At the beginning of May, the FIA was asked by Pirelli if it was possible for it to carry out some tyre development testing with a team, using a current car.
"Within the contract Pirelli has with the FIA as single supplier, there is provision for them to carry out up to 1000km of testing with any team - provided every team is offered the opportunity to do so.
"Pirelli and Mercedes-AMG were advised by the FIA such a development test could be possible if carried out by Pirelli, as opposed to the team that would provide the car and driver, and that such tests would be conditional upon every single team being given the same opportunity to test in order to ensure full sporting equity.
"Following this communication, the FIA received no further information about a possible test from Pirelli or Mercedes-AMG.
"Furthermore, the FIA received no confirmation that all teams had been given an opportunity to take part in this test."
Mercedes team principal Ross Brawn is adamant the test was not secret, seemingly laying some of the blame at Pirelli's door.
"It was up to Pirelli to spread the information. It wasn't up to us, it was their test," said Brawn.
"Pirelli has been asking teams to help them out for 12 months and people haven't been supporting them.
"We made the effort to help them. Nobody else seems to have done that."