Published: 1:46 PM July 2, 2019 Updated: 8:43 AM September 18, 2020

Documents from the European Research Group (ERG), whose subscribers are largely hardline Brexiteers, will finally come to light after an 18-month battle for transparency.

In a case brought by political website openDemocracy, the Independent Parliamentary Standards Authority (IPSA) will share documents that had been provided to it by ERG as part of a compliance review by the watchdog.

The ERG is technically a research service and is bound to IPSA rules which say its activities must primarily support MPs' work, and be neither party-political and nor constitute campaigning. As such, it's not obliged to share its documents, despite being almost entirely funded by the public.

In 2018, an ERG subscription found in MP Bim Afolami's expenses was revealed to cost £2,000.

Despite its stated neutrality, it is widely viewed as a party within the Conservative party, with its known members being ultra-eurosceptics who have outsize influence on Brexit policy. It's not obliged to reveal its full member list, but known members have included Mark Francois, Iain Duncan Smith and Andrew Bridgen, and it is chaired by Jacob Rees-Mogg.

Jon Trickett, shadow minister for the Cabinet Office, has complained to IPSA that it is a "secretive and entirely politically biased group", accusing it of having its own whipping system which it uses to undermine Labour and government ministers.

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But during its review, IPSA had concluded that the documents the group submitted were compliant with the rules, and were "not emotively phrased".

It's these documents that openDemocracy have sought from IPSA since filing a Freedom of Information (FoI) request in January 2018, which was refused until now.

IPSA initially refused to release the scrutiny documents for fear that if they did, "the ERG would not cooperate with IPSA as fully in future", affecting its ability to regulate MPs' expenses in future. It also argued that the ERG had initially submitted the materials under an assurance of confidentiality.

But openDemocracy hit back, arguing that as IPSA is subject to FoI requests it should not have made that assurance to the ERG.

The Information Commissioner argued that "there is clearly a very strong public interest in ensuring that [IPSA's] oversight is as thorough as possible, not only to safeguard the public purse but also as part of the process of continuing to rebuild public confidence in MPs and the expenses they are able to claim".