All LMP prototypes are set to run additional, flashing rain lights from the Le Mans 24 Hours, following Kazuki Nakajima's accident at the Spa World Endurance Championship round.
The FIA and the Automobile Club de l'Ouest, which jointly write the LMP1 and P2 regulations, requested the move ahead of last weekend's Le Mans test day on safety grounds.
The move follows calls from drivers to improve visibility of the cars in the rain after Toyota driver Nakajima fractured a vertebra when he drove into the back of a slow-moving Audi at Spa.
A statement from the FIA Endurance Committee read: "After having seen the LMP rain lights under rain conditions, we feel that their visibility has to be improved.
"Therefore, we would like to implement from [the] Le Mans test day onwards two additional rain lights in the rear transversal plates [either side of the rear diffuser]."
The four LMP1 manufacturers reacted to the directive - issued the week before last - in time for the test day, but LMP2 constructors ORECA and Onroak Automotive, which builds the Ligiers and Morgans, did not.
Porsche LMP1 team principal Andreas Seidl told AUTOSPORT: "After what we have learnt about the incident at Spa, we think it is a good call. Every manufacturer supports it."
Philippe Dumas, who runs Onroak and the sister OAK Racing squad, explained that it had not been possible to implement the change in time for the test day because the marque has nine cars on the grid for Le Mans.
He also questioned whether the move was viable on cost grounds in P2, where flashing rain lights mounted in the wing endplates were introduced for this season.