The FIA have asked McLaren to modify the design of their controversial rear wing amid ongoing intrigue from rivals.
Footage from last week's Azerbaijan Grand Prix appeared to show part of McLaren's rear wing flexing upwards on the straight.
The FIA has been investigating closely after rivals raised questions of its legality.
On Thursday, speaking in a media session ahead of Sunday's Singapore Grand Prix, Ferrari's Charles Leclerc said McLaren's wing was "controversial to say the least."
Red Bull's Sergio Pérez said about the design: "I'm as surprised as you all are. Definitely a massive benefit."
The FIA appeared to draw a line under the issue on Thursday, saying McLaren's wing had passed all flexibility tests and complied with the rules.
The FIA said it was "reviewing data and any additional evidence that has emerged from the Baku GP and considering any mitigating measures for future implementation."
ESPN understands the FIA has asked McLaren to make some modifications to the design, although it is unclear exactly what those are.
McLaren said in a statement: "Whilst our Baku rear wing complies with the regulations and passes all FIA deflection tests, McLaren have proactively offered to make some minor adjustments to the wing following our conversations with the FIA.
"We would also expect the FIA to have similar conversations with other teams in relation to the compliance of their rear wings."
Ferrari boss Fred Vasseur said of the rear wing: "I think it's more than borderline. We all saw the videos and pictures of this, and it's a bit frustrating when, if you remember the situation in Monza, we had five cars in two hundredths of a second, and you move from P1 or P2 to P5 or P6 for two hundredths of a second.
"In Baku we had ten laps in a row when we arrived side-by-side at Turn 1, so you can imagine that we have a bit of frustration."
The controversy follows similar discussions about the flexibility of front wings on the cars of McLaren and Mercedes -- on that occasion, Red Bull and Ferrari both raised questions to the FIA.
Those front wings were also declared to comply with the regulations.
Vasseur clarified the difference between the two.
"I think there is a kind of confusion between what happened with the front wing and the rear wing. The front wing we all agree that it could be a grey area because on the the first paragraph of the TD [technical directive] is saying you can't design part of the car with the intention of the deformation. Intention is difficult to manage.
"The rear wing story is completely different because in the article you also have a maximum deflection, and this is black or white. It's not grey, or dark grey, or light grey. It's black, and black. And for me it's clear."