The rightwing pressure group the TaxPayers’ Alliance has conceded that it illegally sacked the whistleblower Shahmir Sanni for revealing unlawful overspending in the Brexit referendum campaign, in a case that could have a major impact on how lobbyists are described in the media.

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In a development that lawyers have described as “almost unprecedented”, the group has also conceded that it illegally vilified Sanni in the media in coordination with a network of other “linked” organisations.

The alliance has accepted all the allegations Sanni made during his action claiming unfair dismissal, wrongful dismissal, direct discrimination and “dismissal by reason of a philosophical belief in the sanctity of British democracy”.

Significantly, it has also conceded that it is liable for what Sanni’s lawyer, Peter Daly of Bindmans, describes as “extreme public vilification”. Sanni had claimed that it was responsible for a smear attack published by the website Brexit Central, and that it coordinated “derogatory statements” made by the head of Vote Leave, Matthew Elliott, to the media – calling Sanni a “fantasist” (Channel 4) and “so-called whistleblower” (BBC) and claiming that he was guilty of “completely lying” (Sky News)– before an official finding by the Electoral Commission into the conduct of the Brexit referendum.

The disclosure is likely to have far-reaching consequences for the way that broadcasters describe lobby groups. The uncontested claim has stated that the TaxPayers’ Alliance is responsible for Elliott’s Brexit Central website as part of nine “linked” high-profile rightwing “thinktanks” that operate in and around offices at 55 Tufton Street in Westminster and coordinate media and other strategy. [See footnote.]

In Sanni’s case, they also coordinated with Downing Street.

Facebook Twitter Pinterest Shadow chancellor John McDonnell has called on the BBC for transparency over its use of ‘virtual lobbyists’. Photograph: Facundo Arrizabalaga/EPA

The network includes the Adam Smith Institute, the Centre for Policy Studies, the Institute of Economic Affairs and Leave Means Leave. The shadow chancellor, John McDonnell, is calling for a full inquiry into the groups’ funding and said that in the interests of “openness and accountability” the BBC must make clear they are lobbyists, not thinktanks” as they are sometimes referred to.

In March, Sanni revealed to the Observer massive overspending by the official Vote Leave campaign, which has now been found to be in breach of the law by the Electoral Commission. The day before this was published, Downing Street released a statement that revealed Sanni was gay, and the TaxPayers’ Alliance subsequently sacked him from his job running its social media. It has now conceded in full Sanni’s claims and is liable to pay substantial damages.

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Details of the alliance’s relationship with Downing Street and the role of Stephen Parkinson, Theresa May’s political secretary, will now not be heard in court. A separate claim by Sanni against Downing Street is still ongoing. Sanni, who received an award from Gay Times last week, said: “It has proved that the TaxPayers’ Alliance sacked me for speaking the truth. And that there has been a coordinated effort by the Conservative establishment, including the government, to shut me down.

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“The TPA claimed to have lost a donor because of my actions. If they had fought the case in court as we wanted, they would have had to reveal who their donors are. That they were prepared to admit their illegal behaviour on all counts shows how far they are willing to go to protect this information.

“Serious questions must be asked about who is funding them, what their exact relationship is with the government and why are they allowed a platform on national television.”

Chris Milsom, a barrister who specialises in whistleblowing cases, said: “It is incredibly unusual for a respondent to make a complete concession on liability as the respondent has here. To wave a white flag to avoid disclosing documents and giving evidence in court is really unusual. They conceded everything. How does an ostensibly private company come to be working with Downing Street? What is their relationship? Who are their funders?

“If this had been fully ventilated in a public trial we could have found these things out. The effect of these admissions, however, is that Mr Sanni was dismissed both because he blew the whistle on electoral crimes and because of his philosophical belief in the sanctity of democracy. We must now ask: is that an entity that is fit to be on the BBC ostensibly speaking on behalf of all ‘taxpayers’?”

McDonnell said: “We need full transparency in who is operating in our political system and therefore seeking to influence both our elections but also our governmental policymaking. These organisations – even by their names – seek to portray themselves as independent, authoritative research bodies.”

In reality, he said, they were “virtual lobbyists” but never presented as such by the BBC and others.

At the time of publication, the TaxPayers’ Alliance had not responded to the Observer’s request for comment.

• Note added 16 November 2018, updated 21 and 27 November: Only Sanni and the TaxPayers’ Alliance were parties to Sanni’s unfair dismissal claim. BrexitCentral, the Institute of Economic Affairs, the Adam Smith Institute and the Centre for Policy Studies each made contact after publication of this article to deny that they had coordinated to vilify Sanni. Jonathan Isaby, editor of the BrexitCentral website, emailed that the TaxPayers’ Alliance was not responsible for BrexitCentral, and that BrexitCentral’s article about Sanni had been a forensic examination of his claims. A spokesperson for the Institute of Economic Affairs emailed that the IEA was an independent educational charity not a lobbying body.

• This article was amended on 13 and 16 November 2018 to clarify the media outlets on which three quoted comments critical of Sanni were made.