A respected environmental think-tank has declared it has “strong suspicions” that European car companies are artificially fixing laboratory tests of their air pollution levels, just as VW is alleged to have done by US regulators.

VW has admitted systematically cheating on US air pollution tests on its diesel cars for years, the country’s Environmental Protection Agency said on Friday. The violations could lead to fines of up to $18bn (£12bn), as well as criminal charges. Civil lawsuits were reportedly launched over the weekend.

VW’s chief executive, Martin Winterkorn, made a public apology yesterday afternoon for the company’s actions and launched an “external investigation” into the affair.

But the Transport & Environment think-tank said similar behaviour by car companies was highly likely to have been going on in Europe as well. It said it may not have been uncovered here simply because the monitoring system is less robust than in the US.

Greg Archer, a chemist who formerly worked in senior government roles on clean transport, heads Transport & Environment’s Clean Vehicles Team. He said: “We have become increasingly suspicious of the figures on emissions because there is a very wide gap between laboratory performance and performance on the road.”

The US investigation found that VW had installed computer software that could tell when the car was undergoing official emissions testing and would turn on full pollution controls to make its performance better than normal. The controls would be switched back off when the car was being driven on the road, with the result that the cars emitted as much as 40 times the legal maximum, the EPA said. The EPA regulator called this algorithm a “defeat device”.

VW has been in the forefront of advertising diesel cars in the US as fuel-efficient alternatives to petrol.

In the US, diesel sales are a tiny minority of overall car sales – perhaps under 5 per cent. In Europe, diesel makes up closer to half of all sales.

Mr Archer said: “We, and other organisations, have been doing real-world tests and have been shocked at how high the emissions are. We just could not understand how they deliver such low readings in the laboratories.”

He called for European watchdogs to tighten up the monitoring system and bring it up to US levels. “What we have is a system that is just not sufficiently independent,” he said.

The car industry rapidly moved to deny his claims. The chief executive of the Society of Motor Manufacturers and Traders, Mike Hawes, said: “The EU operates a fundamentally different system to the US – with all European tests performed in strict conditions as required by EU law and witnessed by a government-approved independent approval agency. There is no evidence that manufacturers cheat the cycle.”

But he added: “The industry acknowledges, however, that the current test method is outdated and is seeking agreement from the European Commission for a new emissions test … more representative of on-road conditions.”