A parliamentary inquiry into the Volkswagen car emissions revelations was told that "aggressive lobbying" from the car industry poses a "more fundamental conflict of interest."
The comments, by Corporate Europe Observatory (CEO), a Brussels based transparency campaign group, came at a meeting on Thursday.
The Volkswagen scandal was revealed following a study into differences between emissions from diesel vehicles in the US and Europe.
The US Environmental Protection Agency (EPA) discovered that Volkswagen had installed software to manipulate emissions in some of their cars.
Parliament is now holding a series of hearings with key representatives from the European Commission and national authorities.
The special committee consists of 45 MEPs and will investigate the regulatory breakdown in the EU, the illegal manipulation of pollutant emissions tests for cars and the failure to ensure the pollutant limits for cars, set out in EU law, were implemented.
The committee will conclude with a final report early next year summarising its findings and making policy recommendations.
On Thursday, Olivier Hoedeman, from CEO, outlined the findings of its exhaustive research on the "privileged role" enjoyed by industry in policymaking, with a particular focus on the EU.
He stressed that CEO are not technical experts on vehicle testing regimes, but rather on governance and stakeholder influence over decision making.
He gave the findings of its published analyses, "which points to extremely active lobbying by the car industry in Brussels and at national level, as well as close contacting and privileged access to decision making processes."
He told the hearing, "Correspondence released shows that in the year before the real driving emissions (RDE) procedure was voted through the car industry was closely involved in the crafting of the RDE procedure, as well as aggressively lobbying the Commission when it wasn't giving in to European Automobile Manufacturers Association (ACEA) pressure."
He highlighted how ACEA and its members had "weakened the proposal; tried to delay its finalisation; and lobbied around conformity factors."
In March, CEO published another investigation based on a leaked document, outlining ACEA's negotiating strategy around RDE.
"The key points are that ACEA's strategy was based on information gathered during an informal meeting between ACEA and DG Grow.
"Again, much of the lobbying was wrapped up as technical inputs, putting into question the independence of the expertise the Commission relies on and the danger of the over-reliance on expertise when there is a clear economic interest behind it."
He added, "More broadly, our findings pose serious questions over how closely regulators should involve the same industries they are supposed to be regulating in policymaking, and whether this poses a more fundamental conflict of interest."
MEPs on the committee of inquiry said they were particularly keen to reduce the dependency on car industry expertise and ensure the Commission has "far better" access to expertise that is independent from the sector to be regulated.
On meetings with lobbyists, they also pointed to the Commission's guide to staff ethics and conduct, which instructs officials that "a written record of such meetings should be ensured where these contain important information or may involve action by the Commission. Such reports should be registered and filed."
Hoedeman, one of several experts to give evidence at the hearing this week, said, "Based on our experience with freedom-of information requests, DG GROW is not following these rules. This is symbolic for a pattern of very frequent, ongoing and often informal relations between Commission and car industry lobbyists."
He added, "To preserve decision making from undue influence, we believe a more formal approach and clearer rules around contacts with lobbyists are needed."
ECR MEP Mark Demesmaeker, a Vice-Chair of the special inquiry committee, said, "I have been in favour of an inquiry for a long time and Parliament has an important monitoring and research task to fulfil.
"This committee has to deliver full clarity about what went wrong with the application of European emission standards for cars and should also provide solutions for the future. We have to focus on transparency and public health."
He added, "Dieselgate not only violated the trust of consumers, it also put a bomb under the trustworthiness of the European environmental policy.
"Strict environmental standards are merely paper tigers if they are not fully deployed in a robust and rigorous way."