The political group Momentum, who are ardent backers of the Labour leader Jeremy Corbyn, have been placed under investigation by the Electoral Commission over possible spending breaches.
The election watchdog announced that it had started the probe about whether Momentum's spending was "accurate".
Bob Posner, the Commission's director of political finance, said: "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."
Momentum, who have almost 30,000 members, are classed as a registered non-party campaigner by the electoral commission and are being investigated under the Political Parties, Elections and Referendums Act 2000.
The investigation is looking at:
- whether or not Momentum spent in excess of the spending limits for an unauthorised non-party campaigner in the UK Parliamentary general election;
- whether or not Momentum submitted a return that did not include accurate donation information and/or the required declaration stating that the donation return was complete and accurate;
- whether or not Momentum submitted a return that was not a complete statement of payments made in respect of controlled expenditure;
- whether or not Momentum submitted a return that did not include all invoices for payments of more than £200.
The party have grown in influence ever since Jeremy Corbyn was elected as Labour leader, especially during his re-election campaign in 2016.
Posner added: "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."