An investigation has been launched by the UK's democracy watchdog into whether the Jeremy Corbyn-backing Momentum movement breached spending rules at this year's general election.
The Electoral Commission said its probe would look into whether Momentum's spending breached the limits for non-party campaigners in national elections.
It will also consider whether returns submitted by the group, founded in 2015 as a grassroots movement to support Mr Corbyn's leadership of the Labour Party, accurately recorded donations and payments relating to the 2017 campaign.
The Commission's director of political finance and regulation and legal counsel, Bob Posner, 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."