PARIS (Reuters) - French prosecutors have opened a formal investigation into Fiat Chrysler over allegations that the carmaker cheated in diesel emission tests, a judicial source said on Tuesday.

A screen displays the ticker information for Fiat Chrysler Automobiles NV at the post where it's traded on the floor of the New York Stock Exchange (NYSE) in New York City, U.S., January 12, 2016. REUTERS/Brendan McDermid

“I can confirm that a judicial investigation has been opened into aggravated cheating,” the source said.

The source said the Paris prosecutor had opened the investigation on March 15, after the finance ministry’s DGCCRF consumer affairs and anti-fraud body had referred the case to the courts.

A Fiat spokesman said the company took note of the investigation and reiterated that its diesel vehicles fully comply with emission regulations, as confirmed by the Italian Transport Ministry.

The spokesman said the company would continue to collaborate with the authorities on all investigations and was confident the matter would be fully resolved.

Following Volkswagen's VOWG_p.DE admission in 2015 of cheating on U.S. diesel emission tests, several European countries launched their own tests on vehicle emissions.

They found on-road nitrogen oxide (NOx) emissions more than 10 times above regulatory limits - for some GM, Renault and Fiat Chrysler models - and widespread use of devices that reduce exhaust treatment in some conditions.

The French test program, overseen by a commission set up by Environment Minister Segolene Royale, has so far led to four carmakers - Volkswagen, Renault, Fiat Chrysler and PSA Group - being referred to prosecutors.

Fiat Chrysler vehicles were among those that recorded the highest NOx emissions under non-standard testing regimes designed to detect banned engine software cheats.

“The investigation into Fiat is a natural consequence of the Royal commission’s findings,” said Frederik-Karel Canoy, a Paris-based lawyer who represents a group of Fiat owners pursuing related claims against the carmaker in French courts.