The Information Commissioner has ruled that the UK government must name the MPs that belong to the European Research Group, which favours a hard Brexit and is led by Jacob-Rees Mogg MP.
A charity set up by Brexit backer, Arron Banks, has been strongly criticised by the Charity Commission.
The Eurosceptic Conservative MP Steve Baker has been a director and a trustee of the right-wing Cobden Centre since founding it in 2010 but chose not to declare this in the MPs' register of interests.
Concerns have been raised over the charitable status of a free market think tank that has close links to leading Brexit ministers.
A charity run by prominent Brexit donor Arron Banks has been under investigation by the Charity Commission.