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McLaren, Mercedes GP and at least two other teams will have to make modifications to their diffuser designs in time for the Australian Grand Prix, AUTOSPORT has learned, after the FIA told them that it is clamping down on a loophole being used by the outfits.

Discussions took place between the FIA and representatives from four teams over the season-opening Bahrain Grand Prix weekend about the size of starter motor holes in their diffusers.

The size of their starter motor holes was believed to be excessively wide, which, although not in breach of the regulations, was reckoned to be going against the spirit of the rules.

F1's technical regulations state that a hole can exist in the diffuser to allow access for an engine starter motor - although there are no strict dimensions laid down.

Article 3.12.7 states: "A single break in the surface is permitted solely to allow the minimum required access for the device referred to in Article 5.15. [supposed to refer to starter motor, although this is Article 5.16]."

There is no specific definition of what the 'minimum' size is though - so some teams have been using exotically shaped starter motors to allow themselves to feature wide-shaped holes in the diffusers. This concept was pioneered by Brawn GP in 2009.

Such a hole in the diffuser helps create another tunnel for air to flow through - which as well as helping to produce more downforce, also ensures such downforce is more consistent throughout a lap - especially under braking, when the rear of the car rises up and the airflow can stall.

The FIA inspected the diffuser designs in Bahrain and promised to issue a clarification about the matter after the weekend had finished.

AUTOSPORT understands that the FIA has duly sent a note to all teams, laying down strict dimensions for not only a maximum diameter for the holes but also for a maximum projected area.

It is believed that McLaren, Mercedes and two other teams - believed to be Renault and Force India - will now have to make modifications to their diffuser designs in this area prior to the next race in Melbourne.

McLaren team principal Martin Whitmarsh said in Bahrain last weekend that his team was one of many that was under investigation over the matter.

"There are holes in the diffuser for the starter, the hole in ours is no bigger than the one on the championship winning car last year," he said. "And also no bigger than it is on about four other cars."