The US Supreme Court has delivered what could be a landmark judgment on the use of diesel vehicles by fleet operators.

The court decreed that the federal Clean Air Act prohibits the South Coast Air Quality Management District (SCAQMD) regulations that ban the purchase of diesel vehicles by fleet operators.

The news means that diesel engines will now be eligible for operators who control fleets of 15 or more vehicles, as a potential alternative to the use of hybrid or fuel cell vehicles.

In an eight to one ruling the court effectively overturned the judgment of the SCAQMD -a local clean air authority for Southern California - which declared in 2000 that fleets could not use diesel engines, even if they possess emission-controlled diesels.

The challenge to the regulation was brought by the Engine Manufacturers Association (EMA) and the Western States Petroleum Association (WSPA).

US Solicitor General Olson asserted in his presentation to the Court that the US government has the general duty to uphold and defend the primacy of federal law - in this instance being carried out by the Environmental Protection Agency - above and beyond the jurisdiction of states or local agencies to regulate emissions from new motor vehicles.

Diesel cars use platinum catalysts to control harmful exhaust emissions.