F1 teams McLaren, Mercedes, Renault and Force India will all have to race with modified cars in this weekend's Australian Grand Prix.
The four teams have been ordered to modify their diffuser designs by the sport's governing body, the FIA.
The teams were exploiting a loophole in the rules, which allowed them to have a hole in the diffuser to allow them to fit a starter motor. However, it no maximum size of hole was stipulated.
However, the FIA deemed that the teams were using intentionally larger starter motors, which went against the spirit of the rules.
A larger hole in the diffuser helps create another tunnel for air to flow through, helping the car produce more consistent downforce over a lap.
McLaren team principal Martin Whitmarsh said at the Bahrain Grand Prix 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.