Image copyright UK Parliament Image caption The last two chairs of the group, including Suella Fernandes, are now Brexit ministers

Described last September by its then-chair Suella Fernandes as "a group of MPs who are supporting the government to deliver a Brexit which works for everybody", the European Research Group (ERG) has been around for years, but who are they and why don't we know more about them?

We don't know because there is no requirement on the group to publish a list of members.

It has no official role in Parliamentary proceedings, or even within the Conservative Party itself.

It is seen as influential - with reports that there are up to 60 Conservative backbenchers who are members of the group.

But while it has been said that the ERG is a group of "hard-line Brexiteers", BBC research shows that members of the group, past and present, come from all sides of the EU referendum debate.

Late last year Ms Fernandes circulated a letter amongst Conservative MPs that implored government ministers to ensure any transitional period would not become an attempt to keep the UK inside the EU "by stealth".

The letter was put together by Change Britain, a group that evolved from the remnants of Vote Leave, who were the official campaign group for Leave during the EU referendum.

Although reported to have been signed by nearly 40 Conservatives, when Ms Fernandes was later asked by Channel 4 who the members of the group were, she didn't answer, suggesting it was up to individuals to disclose whether they were members of the ERG or not.

In the same interview, Ms Fernandes, who was then a ministerial aide at the Treasury and has since been promoted to minister in the Brexit department, confirmed the ERG "is funded by public money and has been for many years".

This public money comes from an allowance all MPs have available to put into collective research services, and the ERG is not the only such group in Parliament.

BBC research understands the ERG has been in existence since 1992 when Michael Spicer, now Lord Spicer, founded the group while he was the chair of the Parliamentary Office of Science and Technology.

The Maastricht Treaty had been signed by John Major in February that year and Michael Spicer had been one of the rebels who voted consistently against the government.

Image caption Michael Spicer was the group's first chair when it was founded amid the Maastricht battles of the Major years

The new chair, selected after Suella Fernandes' ascension into the ministerial ranks, is Jacob Rees-Mogg.

In between the first and current chair, other known leaders of the group have included: David Heathcoat-Amory, Chris Heaton-Harris and now-Brexit minister Steve Baker, meaning all known chairs of the group were pro-Leave at the time of the EU Referendum.

However it's only since 2010, the first year MPs were required to publish expenses with the Independent Parliamentary Standards Authority, that any of the wider membership can be more easily identified.

MPs who are members of the ERG allocate a small portion of their office allowances to cover the costs of the group's researcher.

From the IPSA records, it appears there are 45 MPs who have claimed fees at one point or another in those intervening years, eight of them are currently serving in the cabinet and another one attends.

Some of those 45 appear to have stopped or put their allowances claims on hold while others did not claim the fee every year.

Cabinet attendees who have claimed expenses for the ERG since 2010:

Image copyright Reuters Image caption Both Leavers and Remainers have been associated with the group

David Davis, Secretary of State for Exiting the EU

Liam Fox, Secretary of State for International Trade

David Gauke, Secretary of State for Justice

Michael Gove, Secretary of State for Environment, Food and Rural Affairs

Chris Grayling, Secretary of State for Transport

Sajid Javid, Secretary of State for Housing, Communities and Local Government

Andrea Leadsom, Leader of the House of Commons (Attends Cabinet)

Brandon Lewis, Party Chairman

Penny Mordaunt, Secretary of State of International Development

Of the Cabinet attendees who have claimed allowances for the ERG, six were pro-Leave while the other three, David Gauke, Sajid Javid and Brandon Lewis, campaigned for Remain. Of the 45 total MPs identified through claims, 33 were pro-Leave and 12 pro-Remain.

However, the allowances records are not the only means by which members of the ERG can be identified.

The group has not been shy in signing and distributing letters and before the attempt to distribute a letter in the Sunday papers in September was aborted, they publicised one sent to Donald Tusk, the president of the European Council.

The letter lists not only the officers as they were in November 2016 when the group "relaunched", but also a list of names of MPs, not all of whom were Conservative MPs, who supported the European Research Group's letter asking for the European Council to guarantee reciprocal rights for resident EU and UK citizens.

The only officer who was a signatory to this letter and hadn't claimed any allowances for contributions into the ERG is the ardently pro-Remain MP John Penrose, who supported several amendments to the EU (Withdrawal) Bill along with fellow Conservative MPs such as Dominic Grieve and Anna Soubry.

Other MPs have revealed themselves to be supporters simply when asked the question, such as the new Deputy Chairman of the Conservative Party, James Cleverly, on an October 2017 episode of BBC's Brexitcast podcast.

Other ERG officers and supporters past and present: