Formula One's governing body has warned teams it will introduce new rear wing flexibility tests next month amid suspicions of rule bending.
Mercedes' seven times world champion Lewis Hamilton flagged up the issue after qualifying for last weekend's Spanish Grand Prix, which he won.
"The Red Bulls are really fast on the straights. They have this bendy wing on the back of their car which they put on today and they gained at least three tenths," the Briton told Sky Sports television.
Red Bull team principal Christian Horner told reporters on Sunday his cars' rear wings had passed stringent tests and were legal.
In a note sent to teams on Tuesday and seen by Reuters, International Automobile Federation (FIA) single seater technical head Nikolas Tombazis said new load deflection tests would be introduced from June 15.
Article 3.8 of Formula One's technical regulations says all components influencing a car's aerodynamic performance must be "rigidly secured to the entirely sprung part of the car" and "remain immobile in relation to the sprung part of the car".
"We have become aware of some cases whereby rear designs which comply with the requirements of articles 3.9.3 and 3.9.4 nonetheless exhibit excessive deflections while the cars are in motion," said Tombazis.
"We believe that such deformations can have a significant influence on a car's aerodynamic performance and hence could be deemed to contravene the provisions of article 3.8."
Tombazis' note did not mention any teams or give specific examples.
"We will be looking out for any anomalous behaviour of the deformation of the rear wing," said Tombazis.
"In particular we will not tolerate any persistent out of plane deformation that may be contrived to circumvent the symmetrical loading applied in the load deflection tests.
"Should we observe any characteristics that indicate exploitation of this area, we will introduce further load deflection tests as necessary."
The FIA can carry out such tests on any part of the bodywork suspected of moving once the car is in motion.
The delay in implementing the new tests until just before the French Grand Prix is to allow teams the time to strengthen their wings, and the first month of the new tests will have a 20% tolerance.
There have been four races so far, with three won by Hamilton and one by Red Bull's Max Verstappen, and France is scheduled to be round eight. Hamilton has a 14 point lead in the championship.
Horner said last weekend that Mercedes boss Toto Wolff had raised the subject of bendy wings in conversation with him, which the Austrian confirmed.