Hispania have threatened to launch a protest in Monaco against Formula 1's latest 'must-have' technology.
FIA technical delegate Charlie Whiting outlawed the use of so-called hot-blown diffusers but then postponed the ban.
But Hispania said if their use was not stopped before Monaco they had no other choice than to make a protest.
"It is clear that the other cars are illegal and it brings the teams a huge advantage," said Hispania team boss Colin Kolles.
Hot-blown diffusers increase downforce and grip by continuing to flow gases out of the exhaust even when drivers are off the throttle.
Hispania, Virgin and Williams are the only teams on the grid not currently using the new technology.
"This is illegal," continued Kolles. "You cannot influence the aerodynamics by hot exhaust gasses or any moveable devices.
"We have studied this very carefully. The point is it is not corresponding to the regulations. It brings the other teams a huge advantage.
"If this is not going to be stopped before Monaco we have no other choice than to make a protest
"I cannot tell you at what stage during the Monaco Grand Prix [we would protest].
"The only reason why we are not considering it [now] is because we were not involved in any sporting decision today."
Whiting postponed the ban after receiving representations from teams using the technology to the effect that they would have had to put significant work into changing their engine maps ahead of the race in Spain.
As a result a decision has been delayed until after the sport's Technical Working Group meet on 16 June, when the issue will be discussed by team engineers and Whiting.
But an official protest may force the FIA to confront the problem before the teams gather in Monaco.
BBC F1 online editor Andrew Benson said the issue was a complicated one.
"What is actually being banned is blowing of the diffuser when the driver is off the throttle - blown diffusers themselves would remain legal," he explained.
"Whiting thinks this is illegal, but decided earlier this week to delay a ban following representations from teams.
"But the fact he thinks that raises the possibility that after a race a team not doing it could protest the nine or 10 teams who are.
"Williams are believed to be the quickest team without the technology."
Red Bull team principal Christian Horner said he was surprised to hear of the intended protest.
Horner said: "I would have thought he [Kolles] had bigger issues than an exhaust blowing at the minute."
Hispania are bottom of the constructors' world championship.