Mercedes and McLaren's Formula 1 bosses have hit out at the decision to delay the implementation of stricter rear-wing flexibility tests until June, with Toto Wolff describing it as "incomprehensible".
Red Bull, however, one of the teams whose wings have been placed under scrutiny by rivals, have stressed that their car passes "all legality tests".
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F1's 'flexi-wing' debate has ramped up in recent weeks after Lewis Hamilton flagged Red Bull's "bendy" rear wing after qualifying on pole for the Spanish GP - with Mercedes adamant it was increasing straight-line speed. The FIA, whose regulations state that bodywork must be immobile, wrote to all 10 teams following that Barcelona weekend to inform them of two new deflection tests from next month's French GP.
But while teams who use more rigid rear wings welcome action, they feel that the fact other rivals can use their current wings for the next two races - and perhaps to a big advantage at the Azerbaijan GP on June 6 - is unfair.
"It is incomprehensible that within four weeks you can't stiffen up a rear wing for the track that is probably most affected by a flexible rear wing," said Mercedes team boss Wolff in Monaco, referencing the Baku circuit with the longest straight on the whole F1 calendar.
He suggested that such a scenario could lead to possible protests.
"It leaves us in no-mans land because the technical directive says that the movement of some rear wings has been judged as excessive," he stated.
"Delaying the introduction for whatever reason leaves us in a legal vacuum and leaves the door open for protests. It's not only us, but probably two other teams that are most affected, maybe more.
"And then probably a protest could end up in the ICA [The FIA's International Court of Appeal] and that is a messy situation that can take weeks before we have a result and we should not have ended in this situation."
McLaren boss Andreas Seidl also hit out at the delay.
"We strongly disagree with the timing of the introduction," he said.
"For us there is no reason why it has been delayed for a further two races and give the guys that have designed their cars in order to have these flexi-wings two more races to have the benefit of it, because from our point of view, what these guys are using is clearly against the regulations.
"We are having dialogue with the FIA at the moment and it's a good opportunity for the FIA to show a strong hand here and not accept this anymore from today onwards."
Red Bull, Ferrari and Alpine bosses admitted they will have to "slightly adapt" their cars to pass the new F1 tests from June 15, while Alfa Romeo are also set to be impacted.
Red Bull boss Christian Horner told Sky F1: "The car passes all the legality tests and that's what the car has to do. So they've changed the test, and that's well within the rights of the FIA to do, so we will have to modify the components to make sure they meet the new test.
"I don't think 'spirit of the rules' exists. It's either black or it's white, it either complies with the rules or it doesn't. And that's how you design your cars."
Horner explained it will cost Red Bull half a million dollars to make the required changes - although said Hamilton's claim that the flexible rear wing could gain Mercedes' F1 rivals six-tenths of a second in Baku was "ludicrous".
"It's difficult to quantify," he said. "But I'd be surprised if it was a tenth."
Mercedes and Red Bull are currently locked in a gripping, and close, F1 2021 title battle.
Wolff criticises tests, Horner points finger back at Mercedes
Sky F1's Ted Kravitz compared the flexi-wing to a limbo, with the rear wing on certain cars at the last race in Barcelona moving down on the straight before popping back up to generate downforce in the corners.
Wolff said Mercedes had, ironically, "been left in limbo" having been chasing action from the FIA on the wings since last summer, but criticised the measures they are introducing next month. He described the tests, which will allow some wing rotation, as "half-baked", claiming they will allow wings to "soften and bend".
"We're not satisfied at all," he told Sky F1. "It's a frustrating process.
"We've been flagging the topic, not only us but other teams, from August last year with no reaction. And now we have a solution halfway which means we can make our rear wing flexible rather than having an immobile rear wing.
"It's really half-baked."
Wolff added: "We will need to modify our wing. We need to soften it. Our wing is extremely rigid complying to the famous Article 3.8 [in the FIA regulations] that says it must remain immobile."
Horner, however, pointed the finger back at Mercedes and suggested that they could be vulnerable to car changes if the FIA were to clamp down on wing movement at the front of the car.
Footage of the Mercedes' W12 from the Emilia Romagna GP appeared to show the team's front wing flaps moving up and down.
"It's a competitive business," said Horner. "You've only got to look a bit of the footage from Imola, when we're talking about flexibility, I think it's very unfair to point it at one direction at the rear of the car when you should also perhaps be looking at the front of the car.
"I think if the camera was facing the other way on our rivals' side they'd be having the same discussion with you, 100%."
To which Wolff responded: "We would wish it would be stiffer because that would give us more performance on the front end. Anybody has an idea... we are open for it."