The Dutch advertising watchdog will on Tuesday censure Shell and Exxon for claiming that natural gas was “the cleanest of all fossil fuels” in an advert earlier this year.

It will be the second time this summer that the Netherlands advertising standards board has ruled against the fossil fuels industry, after it slapped down Statoil in June for calling gas a “clean energy” and “low emissions fuel”.

In a bid to pre-empt the latest ruling, online versions of the advert were changed a few weeks ago to redefine gas as “the least polluting fossil fuel”.

The Dutch watchdog waived punitive action against the NAM company, which is part-owned by Shell and Exxon, in that light.

Paul de Clerk of Friends of the Earth Europe, which co-filed the complaint with Milieudefensie, said: “This clear ruling by the advertisement standards board is of great importance. Time after time we see how oil and gas companies are misleading citizens and politicians.

“They want us to believe that gas is clean and they support the transition to renewable energy. Behind the screens we see how the same companies lobby against this transition. To prevent catastrophic climate change we need to end the dependency on all fossil fuels – including gas. ”

The issue is highly contentious, after a Trump administration push for gas to be described as a “clean technology” at a G20 summit in July split the meeting.

The US disassociated itself from the final communique, inserting text into the document saying that it would help countries “access and use fossil fuels more cleanly and efficiently.”

But gas is seen by many policy-makers as a “bridging fuel” to a renewable-powered future and the EU’s research wing has also described it as a low carbon energy source, making about £100m available for research grants on that basis.

Natural gas emits around half as much carbon dioxide as coal when burned, although this is still far more than from renewables.

But the picture is less clear when it includes methane – a short-lived but much more potent greenhouse gas – which one recent study found was being emitted by gas plants at levels up to 120 times higher than reported to US regulators.

The ruling by the Dutch Advertising Code Authority, which the Guardian has seen, found that “the absolute term ‘cleanest fossil fuel’ is not in line with the MRC [Dutch advertising code].”

“It is suggested that fossil fuels can be clean in that they do not cause environmental damage. It is firm – and recognised by NAM – that that suggestion is not correct.”

Antonetta van Beem, a spokeswoman for NAM, said her company’s decision to change the advert was not an admission that the original had been misleading.