Ferrari have been ordered to change their mirrors by the FIA ahead of this weekend's Monaco Grand Prix.
The Italian constructor changed the design of the mirror ahead of the Spanish Grand Prix last weekend, with a supportive arm above the mirror itself, which improved the aerodynamic nature of the fixture.
Subsequently Ferrari were criticised for the addition, which the FIA had deemed legal at the time, but have since changed their position on the matter. The new regulations state that all mirrors must remain mounted to the halo, but within the supportive winglet.
The FIA released a statement to the press laying out the new regulations: "Whilst the FIA accepts that teams will legitimately design the mirrors, housings and mountings to minimise any negative aerodynamic effects they may cause, we believe that any aerodynamic benefits should be incidental, or at least minimal.
The Monaco Grand Prix will be shown live on eir sport 1 on Sunday, 27 May [start time 2.10pm Irish time].
"In order to ensure this is the case all mountings must:
"a) Provide a meaningful structural contribution to the mounting system. If you use more than one mounting you may be asked to satisfy us, by way of a physical test, to demonstrate this.
"b) Be mounted to the lower and/or inboard surface(s) of the mirror housing.
"As the criteria for determining the eligibility of a mounting are to some extent subjective, the FIA would be available to discuss the legality of a new design before you introduce it in a race, to avoid wasting resource, time or money."
"For the sake of clarity, the various provisions made in TD/014-18 still apply for halo-mounted mirrors.
"The FIA expect to have full compliance with the present technical directive by the next race.
"The FIA do acknowledge that the rules currently in force with regard to mirrors are not perfect, and will strive to propose a more complete set of rules in terms of mirror position, mountings, visibility, etc. in the near future, with the aim to get a unanimous support for such changes for 2019."
The move from the FIA is one that was predicted by many in the wake of the Spanish Grand Prix, with race director Charlie Whiting among those to state a ban was imminent.
"The interpretation hinges on whether we think that's a mounting or not. We somehow think not," Whiting said after the race in Barcelona.
"They think it contributes to the rigidity of the mirror. I doubt they would be there if there wasn't a measurable aero advantage, but these days that doesn't have to be big.
"We sent a technical directive a few weeks ago in response to a number of questions from other teams about the principle of mounting a mirror on the halo was acceptable.
"We answered yes, and gave a few stipulations, one that it has to be a mounting.
"It's just a matter of interpretation and such a tenuous interpretation is not something we're happy with."