Shanghai: Mercedes' controversial "F-duct" rear wing was back in the spotlight on Thursday when the Lotus team launched an official protest ahead of the Chinese Grand Prix in Shanghai.

The car has been the subject of fierce debate and speculation because of the rear wing, which rivals say gives the team an unfair disadvantage by boosting straight-line speed.

But it had been cleared for Sunday's race, before the protest, just like it was prior to the first two races of the season, in Australia and Malaysia.

The "double-DRS" device, as it has been dubbed, supposedly uses the DRS activation mechanism to open holes in each of the rear wing's endplates.

These holes channel airflow through the chassis to the nose, where it exits and is used to stall the front wing, cutting drag and boosting top speed.

Debate has focused on whether this constitutes a driver-operated aerodynamic device.

The protest by Lotus is believed to have been filed under Article 3.15 of the FIA's (the ruling body Federation Internationale de l'Automobile) 2012 technical regulations.

Article 3.15 states: "With the exception of the parts necessary for the adjustment described in Article 3.18, any car system, device or procedure which uses, or is suspected of using, driver movement as a means of altering the aerodynamic characteristics of the car is prohibited."

It is not only Lotus who have grave doubts over the legality of the rear wing, Red Bull, among others, have also expressed concerns.

Despite the supposedly unfair advantage, Mercedes have just one point on the board from two races - courtesy of Michael Schumacher's 10th place in Malaysia.