Fiat Chrysler diesel vehicles spewed pollution as much as 20 times the legal limit, according to testing by the same researchers who first recorded the excess emissions in Volkswagen's diesel cheating scandal.

The results shed new light on the United States Justice Department's allegations in a civil lawsuit last month that Fiat Chrysler Automobiles has used illegal "defeat devices", software that helps evade emissions tests.

It comes amid growing concerns about the ability of diesel engines to satisfy US emissions limits and the extent to which automakers navigated around them.