It is alleged that the car produced NOx emissions of 585mg/km, rather than the 80mg/km it was understood to produce, when tested on the roads in December by a research centre commissioned by the European Commission.
In addition, under laboratory conditions, outside of the normal test temperatures of 17-23deg celcius, three times the permitted NOx emissions were produced, claims the French newspaper.
It is reported that the PSA Group, parent company of Citroën, DS and Peugeot, has sent its vice-president, Christian Chapelle, to explain the results to the European Commission.
Lucia Caudet, Commission spokesperson for internal market, industry, entrepreneurship and SMEs (small and medium-sized enterprises), said: "In the third quarter of 2016, the Commission's in-house Joint Research Centre (JRC) took a number of emissions measurements on four cars in the laboratory and on the road. The JRC took measurements on three diesel cars and one petrol car. These cars were selected randomly and they were of different models, years, mileage and mostly rented, hence the measurements are not indicative of particular concerns nor representative of entire fleets.
"A discrepancy between NOx emissions measured in the laboratory and in real driving conditions is not necessarily indication of illegal behaviour. Under the current legal framework, it is for the national authority that type-approved or certified the car to determine whether illegal defeat devices were used.
"On 22 December, the Commission transmitted the information on these measurements to the relevant national authorities and to the EP Inquiry Committee on car emissions. It will be up to the national authorities to publish the results of their national investigations including fully representative and analysed information on entire car fleets."
So far, French environment minister Ségolène Royal has not commented on the alleged findings. However, earlier this week, following reports that Renault is under investigation for emissions discrepancies, she suggested that Citroën, DS and Peugeot could all face scrutiny regarding their cars' emissions from the French government.
Royal commened that "there could be other investigations," and that "[Renault] far exceeded the permissible standards. This is also the case of other manufacturers, to a different extent."
A PSA spokesman couldn't add any response to the possibility of investigation, but did clarify that "PSA will continue to cooperate fully with the commission regarding all the results".
Regarding the C4 Cactus's emissions, PSA later said: "We are surprised by these results, which were not communicated to PSA and which protocol is unknown to us. Especially since similar vehicles with the same engine have been tested by the French authorities and highlighted consistent results, in particular regarding the real-world D3 test."