Corbyn supporting Momentum have come under investigation by the Electoral Commission to see if they broke finance rules in the 2017 General Election.
The Elections watchdog says it will probe whether or not Momentum’s returns included accurate donation information.
Momentum said: “Much of the Electoral Commission investigation refers to a series of administrative errors that can be easily rectified.”
They added that they would fully comply with the investigation.
The organisation was set up in 2015 to help support fringe left-wing Jeremy Corbyn leadership bid in 2015 and now campaigns for labour and varying degrees of left wing policies.
They registered as a non-party campaigner in 2017, during the snap general election.
Bob Posner, the Electoral Commission’s director of political finance, regulation and legal counsel, said: “Momentum are a high profile active campaigning body.
“Questions over their compliance with the campaign finance rules at June’s general election risks causing harm to voters’ confidence in elections.
“There is significant public interest in us investigating Momentum to establish the facts in this matter and whether there have been any offences.”
The Electoral Commission say they are investigating related to the 2017 campaign and whether or not Momentum accurately recorded donations and payments during that time.
It would also consider whether a return failed to include all invoices and payments of more than £200.
Mr Posner said: “Once complete, the commission will decide whether any breaches have occurred and, if so, what further action may be appropriate, in line with its enforcement policy.”
Under rules in place since 2000, non-party campaigners who wish to undertake “targeted spending” – intended to influence people to vote for one particular registered political party or any of its candidates – have to do so within prescribed limits.
The limits – £31,980 in England, £3,540 in Scotland, £2,400 in Wales and £1,080 in Northern Ireland – applied during the regulated period 9 June 2016 to 8 June 2017.
Registered non-party campaigners are only entitled to spend above these limits if they have the authorisation of the political party they are promoting, the commission said.