French prosecutors have opened a formal investigation into suspected diesel emissions test-cheating by PSA Group, the maker of Peugeot and Citroen cars, reports Reuters.

The company is being investigated for alleged consumer fraud offences, a judicial source said on Monday.

In February, Paris-based PSA became the fourth carmaker to be referred to prosecutors by France's DGCCRF consumer fraud watchdog over suspected diesel test manipulation in the wake of the Volkswagen "dieselgate" scandal.

PSA has denied any wrongdoing and repeated on Monday that it complies with all laws in the countries where it operates.