An investigation has been launched into whether the Jeremy Corbyn-backing Momentum movement breached spending rules at this year's general election.
The UK's democracy watchdog, 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 accurately recorded donations and payments relating to the 2017 campaign.
The Momentum movement was founded in 2015 as a grassroots movement to support Mr Corbyn's leadership of the Labour Party.
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."
A Momentum spokesman said: "Much of the Electoral Commission investigation refers to a series of administrative errors that can be easily rectified.
"Momentum put a lot of effort and resources into detailed budgeting and financial procedures during the election to ensure full compliance.
"Our election campaign was delivered on a low budget because it tapped into the energy and enthusiasm of tens of thousands of volunteers across the country.
"We have a good working relationship with the Electoral Commission, and will fully comply with the investigation going forward."