European Union carmakers may be submitting "inflated" figures for their vehicles' carbon dioxide emissions to soften future targets for reducing the greenhouse gas, European Commission documents showed today.

Authorities are currently switching over from an older emissions testing procedure known as NEDC to a new one called WLTP.

During the changeover, "emission values officially declared by manufacturers may be inflated," Commission officials wrote to senior members of the European Parliament's environment committee and EU Council president Austria.

Declaring higher emissions values now "would cause an increase of the 2021 WLTP targets... the targets for 2025 and 2030 would also be weakened," the officials explained.

The new allegations differ in important details from the "dieselgate" scandal that has rocked the car industry since 2015.

In that case, Volkswagen admitted to manipulating millions of cars to appear to emit less harmful nitrogen oxides (NOx) in lab tests than they were in real on-road driving.

Other manufacturers like Mercedes-Benz parent Daimler have since fallen under suspicion.

Without naming any individual companies, the Commission now alleges manufacturers are cheating in two ways to undermine its target of reducing CO2 output 15 per cent by 2025 and 30 per cent by 2030.

First, they are declaring emissions of CO2 on average 4.5 per cent higher than actually measured values, with reporting for some models as much as 13 per cent higher than measurements.

Secondly, officials saw "some evidence" manufacturers are configuring vehicles differently for the WLTP and NEDC tests, aiming to maximise emissions on the new tests - thus softening their future targets - while minimising them on the older regime.

Contacted by AFP, German high-end carmaker BMW pointed to a statement from the country's Federation of the Automotive Industry (VDA), which said that "German carmakers are doing everything they can" to reduce emissions to 95 grammes of CO2 per kilometre by 2021.

"Reporting higher emissions would be counterproductive," the federation claimed.

Meanwhile a spokesman for France's Citroen and Peugeot manufacturer PSA said there was "no cheating" at the group, adding that it publishes all its emissions data online.

Commission officials suggest revising EU regulations to make sure measured, rather than reported, CO2 emissions under WLTP are used to define future targets.

The EU's executive body also wants to overhaul legislation on emissions targets, adding amendments including one to make sure carmakers use vehicles configured the same way for both the old NEDC and new WLTP tests.