The European Research Group is one of the most powerful forces in British politics – and also one of the most secretive. The cadre of pro-Brexit Tory MPs, led by Jacob Rees-Mogg, refuses to even say who its members are.

But now documents produced by the ERG and given to Parliament’s standards watchdog will be made public for the first time. The move follows a ruling by a judicial body in response to a long-running openDemocracy investigation.

The ERG played a pivotal role in forcing Theresa May from Number 10 and is now widely seen as backing Boris Johnson. It has received hundreds of thousands of pounds in public funds for its work. Many of its papers are for internal consumption but previous public ERG reports have been branded “dreamland stuff”.

The ERG is part-funded by subscriptions paid out of MPs’ parliamentary expenses. As a consequence the group has to supply samples of its research for scrutiny to the Independent Parliamentary Standards Authority to ensure public money is being properly spent and not used for party political campaigning.

IPSA, the body set up in the wake of the MPs’ expenses scandal, has continually refused to make public the ERG’s documents. But after a year-and-a-half-long openDemocracy investigation, an information rights tribunal has ruled that IPSA must release material that the ERG has given it. Transparency campaigners and opposition politicians have welcomed the ruling.

In January 2018 openDemocracy asked IPSA to release the ERG research it held, under freedom of information law. But the parliamentary watchdog refused. An appeal to the Information Commissioner’s Office was also rejected.

But last month the information tribunal ruled that the ERG research materials must be made public. Releasing them would “further transparency, accountability and public trust with respect to the working of Parliament”, the tribunal said.

“Dreamland stuff”

The ERG’s research has been heavily criticised in the past. A nineteen-page paper on the Irish border published last year was widely dismissed by trade experts and described by the Irish government as “dreamland stuff”.

Previously openDemocracy has revealed that the ERG breached IPSA’s rules in one of its research briefings. The problem was a statement which said that Labour’s decision to vote against the Withdrawal Bill in 2017 was “irresponsible, a breach of trust with their voters and a vote to create chaos”. IPSA told the ERG that it should “avoid using similar language in the future”.

Its words have influence. The ERG has been called a party within a party, going way beyond what is considered to be a typical parliamentary research service. Coordinating its activities through WhatsApp, it has emerged as a major force in the Brexit process, with senior ERG figures pushing for a ‘no deal’ departure from the European Union.

The ERG is widely seen as backing Boris Johnson’s leadership bid but many political commentators believe the group could seek to topple either Johnson or his rival, Jeremy Hunt, as prime minister if he does not follow through on promises to leave the EU by October 2019.

The group does not publish lists of its members, but earlier this year the Department for Exiting the European Union was forced to reveal to openDemocracy details of a ‘senior’ group within the ERG, which included members of the House of Lords, an MEP and a dark-money-funded lobbyist.

The ERG also accepted a donation from the Constitutional Research Council – a secretive organisation that channelled a controversial £435,000 donation to the DUP’s Brexit campaign. The ERG does not publish its accounts.