The UK’s Electoral Commission is to visit the offices of The Brexit Party to probe how it receives funding.

Campaigning in Glasgow, former Prime Minister Gordon Brown accused the pro-Brexit party of receiving large donations via small “undeclared, untraceable payments,” a statement Nigel Farage dismissed as a smear. Labour Shadow Chancellor John McDonnell also asked for an inquiry into the funding of the Brexit party.

Under campaign legislation, donations below £500 do not have to be declared, while any contribution in excess of that must have a “permissible donor” listed on the UK electoral roll or a business operating in the UK.

Farage claims that most of the Brexit party’s funding comes from 110,000 people making a £25 donation, a feat he boasted other parties cannot emulate. Unlike the Conservatives and Change UK, Brexit party donors are not required to give their name and address before contributing £500 or more, which theoretically allows the same donor to make several payments below the legal threshold. In theory, that is illegal, but it is hard to trace payments.

Farage is also under fire for receiving payments from his major donor, Arron Banks, while still a member of the European Parliament, without declaring the payments. He is said to have received over £450,000 and has been referred by Green MEP Molly Scott Cato to the European Anti-Fraud Office.