London - Halo-mounted wing mirrors will remain legal in Formula One but Ferrari must change the fixings used in Spain last weekend before next week's Monaco Grand Prix.
The governing International Automobile Federation posted a clarification to teams on Thursday saying they could legitimately design mirrors to minimise any negative aerodynamic effects but benefits should be minimal.
In the interests of better visibility, teams were told in April they could fix mirrors to the halo head protection device that encircles the cockpit and that became mandatory this season.
Ferrari's solution pushed the boundaries, with winglets above the mirrors that were fixed to the halo, and raised suspicions they were designed to produce an aerodynamic advantage.
'Meaningful structural contribution'
The FIA said all mountings must "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."
The governing body suggested discussing any new design with the FIA before it was introduced at a race "to avoid wasting resource, time or money.
"The FIA expect to have full compliance with the present technical directive by the next race," it added.
Monaco, which Ferrari dominated in 2017, is next up with plenty of tight and twisty corners putting a premium on high aerodynamic downforce.
Ferrari, second in the championship, was allowed to race in Spain with the mirror arrangement because it was not seen as a clear breach of the regulations but something that needed clarifying.