The Electoral Commission has just announced it is investigating Momentum to see if the group "breached campaign finance rules in relation to spending".
The body wants to establish whether the group exceeded spending limits for an unauthorised non-party campaigner in the UK Parliamentary general election; whether the return it submitted about donations was accurate or complete and whether it included all invoices for payments of more than £200.
"It is possible that during the course of the investigation, the Commission will identify potential contraventions and/or offences under PPERA other than those set out above," the body said.
According to Commission rules, non-party campaigners must stay within annual "targeted spending" limits - which range from just under £32,000 in England to slightly more than £1,000 in Northern Ireland.
Registered non-party campaigners are only entitled to spend above these limits if they have the authorisation of the political party that they are promoting, when they are able to spend up to the limit authorised by the party. In that instance, any additional spending would count towards the party’s national spending limit.
Bob Posner, director of political finance and regulation and legal counsel at the Commission, 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.
“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."
A Momentum spokesperson said the group had maintained a low budget during the campaign "because [we] tapped into the energy and enthusiasm of tens of thousands of volunteers across the country".
He added: “Much of the Electoral Commission investigation refers to administrative errors that can be easily rectified. We have a good working relationship with the Electoral Commission, and will fully comply with the investigation.”