Andrea Dovizioso says Yamaha's new fairing in the Valencia MotoGP test shows banning winglets last year was "a farce", and was only enacted to harm Ducati's progress.
MotoGP banned winglets at the end of 2016 on the basis of them being considered to be too dangerous - a move that mainly affected Ducati, which pioneered the devices in 2015.
During the 2017 season, manufacturers compensated for the lack of winglets with aerodynamic fairings, with Ducati developing the most aggressive version.
Yamaha tested its own radical-looking fairing at Valencia, having only used milder evolutions of the standard bodywork in the season, featuring sharp edges similar to the old winglets.
According to Dovizioso, fairings are now more dangerous than the winglets, which shows that banning the latter in the first place was only done to hurt Ducati and not for safety reasons.
"I don't want to create controversy, but there is only one possible interpretation," Dovizioso said.
"We were told that the wings are forbidden because they are too dangerous, and these fairings today are more dangerous than those wings.
"I agree with the current regulations, but [the new fairings] prove that the reasons taken back then were a farce.
"So it shows that the real reason of the ban of the wings was to limit the progress of Ducati."
Vinales: New fairing an improvement
Yamaha rider Maverick Vinales said the new fairing made the bike better, and reckoned it is not against the rules.
"I don't think it's illegal," said Vinales. "Aprilia is doing the same, and Ducati, so I don't know if it's according to the rules.
"Finally looks like a winglet, but they just... take it out on the fairing, but on the bikes looks the same. Honestly this is about the decision of Dorna, I agree what they decide, I agree.
"It feels good, it feels really good. Actually we improve, and in one side here, in Valencia, we know for sure that the fairing, it's much better."
Additional reporting by Oriol Puigdemont