The new Ford Mustang that has dominated the start of the 2019 Supercars season will undergo changes to its rear wing and undertray as part of a parity aerodynamic adjustment.
Nine of this year's 10 races so far have been won by the Mustang.
Supercars maintains strict technical parity between its cars through homologation, which governs factors such as aerodynamics and engine performance.
In-season tweaks are not overly common, although recent examples include the use of composite bodywork for the Ford and Holden teams after the introduction of the ZB Commodore in 2018, as well as the centre of gravity adjustment made to the Mustang and Commodore last month.
The Mustang's controversially large rear wing endplates will be made smaller, while the gurney flap will be reduced in height and the undertray in length.
The decision follows analysis of data from the recent Phillip Island round along with CFD assessment, with the changes set to be in place for the next round at Perth's Barbagallo track at the start of May.
Mark Rushbrook, global director of motorsport for Ford Performance, argued that the Mustang's result were down to the teams running it rather than technical advantages.
"The Mustang is an advanced, state-of-the-art Supercar, designed and built within the rules of the series," he said.
"We are disappointed that we have had to make changes to the cars, however we respect the Supercars technical department and will comply.
"We will make these changes ahead of the next round so we can quickly understand the affect they'll have on Mustang to give us the best chance of maintaining our pace.
"Mustang is run by some of the best teams in the series and that's not technical parity, it is the sporting performance of the teams that race the car."
Rushbrook added that he does not expect any further changes to the Mustang's aero, despite ongoing talk regarding overseas windtunnel testing.
"Our car was signed off and homologated by Supercars ahead of the 2019 season, however whilst we understand these changes are in the interest of the sport, we expect to run the rest of the season on track unchanged from this specification," he said.
According to Supercars CEO Sean Seamer, the process of determining the changes has been transparent.
"The Supercars technical department has worked through the data available and requested these changes from homologating team DJR Team Penske and Ford Performance in a transparent manner," he said.
"The Mustang is a world class racecar and we respect the work that has gone into developing the package. Despite passing VCAT any incoming car needs to meet the incumbents.
"DJRTP and Ford Performance understand that technical parity underpins the success of the sport and accept the changes."
The parity adjustment comes after significant debate over the Mustang's aero efficiency, particularly in high-speed corners.
While the car's straightline aero numbers were signed off by rival teams during the VCAT homologation testing, its performance on circuits such as Albert Park and Phillip Island has led to speculation that the car generates significantly more mid-corner downforce than the Holden or Nissan.
With yaw performance difficult to assess in the current VCAT process, there has been talk about windtunnel testing - currently banned by Supercars - being employed to help properly judge aero numbers.
Alack of suitable tunnel in Australia means that would need to be done overseas, making it an expensive prospect.
The aero change is the second parity adjustment that has been thrown at the Mustang; following the Albert Park round it was the worst affected by a mandated centre of gravity change, in which 28 kilograms of ballast were relocated to the roofline of the car.
Holden's ZB Commodore underwent a 6.7-kilogram redistribution, while the Nissan Altima was unaffected by the CoG ruling.
Reigning Supercars champion Scott McLaughlin has won seven of the races so far this season, while his DJRTP team-mate Fabian Coulthard and Tickford's Chaz Mostert have a win each, and Triple Eight Holden driver Shane van Gisbergen has the sole non-Mustang victory to his name.