More than one in five attenders at EU events on regulating big tech companies did not disclose links to the industry when applying to take part, according to transparency campaigners who say hidden networks are distorting public debate.

Researchers at three NGOs analysed nearly 4,000 registrations at European Commission workshops organised earlier this year to test companies’ compliance with the Digital Markets Act (DMA), a law to curb anti-competitive behaviour.

The DMA, which came into force in March, designated Google’s parent company, Alphabet, Amazon, Apple, ByteDance, which owns TikTok, Meta and Microsoft as “gatekeepers”, meaning they have to comply with EU rules to ensure fair competition. These large companies, for example, cannot treat their own products and services more favourably than competitors, or prevent users from uninstalling pre-installed software.

Apple, Alphabet and Meta are already being investigated for potential breaches of the DSA and face hefty fines if found guilty. Apple and Meta have said they comply with the act, while Google said it had made changes and would defend its approach.

To check compliance the commission organised six workshops last March, one per company. All attenders were asked to describe any affiliation with the company on the agenda. Researchers, however, found that 21% of attenders – variously working at law firms, lobby companies, trade associations and thinktanks – did not refer in their applications to their links to the companies being discussed.

Margarida Silva, one of the report’s authors who works at the Amsterdam-based NGO Somo, said: “Public workshops can be a great way to test big tech’s compliance with new rules. However, unless there are strict disclosure and conflict of interest safeguards implemented, they can also easily be distorted and taken over by lawyers, lobbyists and experts hired by or funded by big tech.”

The report, which was co-authored by the Corporate Europe Observatory in Brussels and Germany’s LobbyControl, comes after a former EU official wrote in the Guardian in 2022 that the Brussels policy community was “insidiously” in the grip of the big tech corporate class.

The report was based on analysis of online registration forms where participants were invited, but not required, to register any link with the company being discussed that day. Researchers then cross-checked the answers with public information, including company websites and the European Commission’s transparency register, a database of organisations seeking to influence EU decision-making.

Researchers counted representatives from 53 lobbying and public affairs firms at the workshops. These included FleishmanHillard, which has represented Meta and Amazon, as well as Flint Europe, which has worked for Amazon, Apple, Alphabet, Meta and Microsoft. Neither company – nor any of their competitors – disclosed these links, the report said.

A spokesperson for Fleishman-Hillard confirmed that “we followed online the publicly streamed DMA workshops earlier in the year” and sent a link to the firm’s entry on the transparency register revealing its 2023 clients.

Representatives from trade associations, including DigitalEurope and the Computer and Communications Industry Association (CCIA), also took part in the workshops but failed to declare their links with the gatekeepers, researchers said.

DigitalEurope declined to comment, while a spokesperson for the CCIA rejected the claim. “Both of my colleagues who attended made it explicitly clear during their presentations that CCIA Europe represents both gatekeepers and access seekers,” the person said, making a reference to smaller tech companies. The spokesperson added that the CCIA was “very open about its membership; we consistently provide an up-to-date overview of all members on our website” without hidden affiliations.

The NGOs also turned the spotlight on the International Center for Law and Economics (ICLE), which describes itself as a privately funded research organisation. The ICLE was represented by a “senior scholar” at the Amazon event, who asked a question, without declaring at any point that their organisation was funded by Amazon and Meta, the report said.

An ICLE spokesperson said: “Out of respect for the privacy of our donors, we do not disclose or discuss their contributions, which come in the form of unrestricted, general support.” The ICLE had decided to register on the EU transparency registry and would make disclosures there “out of an abundance of caution”, the person said. The ICLE appeared on the transparency register on 29 October, after being contacted by the Guardian in relation to the NGOs’ report.

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Following the Guardian’s questions to ICLE, an Amazon spokesperson offered an unsolicited comment, without confirming whether the company funded that organisation. “We work with organisations like trade associations and thinktanks, and communicate with officials at the European institutions. We update our entry in the EU transparency register in line with the guidelines.”

More than 1,000 registrations were from lawyers, but about one-third of the delegates failed to disclose their firms were working or had until recently worked for gatekeepers, the report said. The NGOs reported that the magic circle law firm Freshfields, reported to represent Apple and Meta, had the highest number of registrations with 81. Ten Freshfields employees signed up for the Apple and Meta workshop, yet only one disclosed working for Apple. Similarly, 10 lawyers from Skadden attended the ByteDance event without disclosing that their firm had acted for the TikTok owner in an ultimately unsuccessful challenge against being designated as a gatekeeper.

Campaigners highlighted “the immense asymmetry of resources” between tech companies and the commission. They counted 80 dedicated employees at the commission’s DMA unit, v 106 employees working for gatekeepers, supported by 282 lawyers and lobbyists.

Max Bank, a campaigner at LobbyControl, described the situation as “a classic David versus Goliath scenario”, adding: “Unless the EU strengthens its enforcement capabilities, the promise of the DMA to limit the power of big tech risks falling short.”

Tommaso Valletti, a professor of economics and former chief economist at the European Commission’s competition department, said he was “very concerned” about the lobbying activities of big tech and other powerful corporations. “Resources are incredibly asymmetric, and it’s very, very difficult to hear other views.”

Valletti was not involved in the research, but provided a statement to the NGOs, which said: “Conflicts of interest and lack of transparency have contributed to a breakdown in the distinction between expertise and advocacy in competition policy and regulation. They need to be fixed upfront in order to have a proper discussion about the digital future of Europe.”

Flint Europe, Skadden and Freshfields did not immediately respond to requests for comment.