ROME (Reuters) - Italy's anti-trust agency said on Monday it had fined German carmaker Volkswagen 5 million euros ($5.54 million) for allegedly misinforming car buyers about diesel emissions results.

The Italian watchdog said it would impose the highest fine in its power on Volkswagen, which it said had marketed diesel-powered cars that had been tested for polluting emissions using a software that gave artificially low results.

Volkswagen (VW) said it plans to challenge the fine at an administrative court.

VW and its Italian division "have fully cooperated with the utmost openness and transparency" to help ascertain the facts, the carmaker said and "are certain that the IAA-decision presents well-founded grounds of appeal".

Europe's largest automaker is caught up in its biggest-ever corporate scandal and faces criminal investigations in the United States, South Korea and elsewhere over cheating in diesel emissions tests.

(Reporting by Isla Binnie; Additional reporting by Andreas Cremer; Editing by Steve Scherer and Alexander Smith)