World champion Sebastian Vettel told Sport Bild: "It's funny, when the rules prohibit testing, and then a team goes testing."
He said it is obvious Mercedes have obtained an unfair advantage.
"Every lap is certainly an advantage," Vettel insisted.
The boss of F1's other Red Bull-owned team, Toro Rosso, agrees: "This is the most blatant rule violation of recent years," said Franz Tost.
Mercedes argues that it cleared the test with the FIA, while Pirelli says its contract with the governing body allows 1000km tests with the F1 teams.
But, with the FIA now investigating, Red Bull's Dr Helmut Marko is resolute: "We will not let up."