An investigation has been launched by the UK's democracy watchdog into whether the Momentum movement, which backs Jeremy Corbyn, 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 accurately recorded donations and payments relating to the 2017 campaign.
Bob Posner, the Commission's director of political finance and 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.
"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."
Momentum was founded in 2015 as a grassroots movement to support Corbyn's leadership of the Labour Party.
It boasts a network of more than 23,000 members, 150 local groups and 200,000 supporters, many of whom took part in campaigning around the 2017 election.
Electoral law imposes strict spending limits on non-party campaigners seeking to influence people to vote for one particular political party or any of its candidates.
Spending is limited to £31,980 in England, £3,540 in Scotland, £2,400 in Wales and £1,080 in Northern Ireland for the regulated period, which in this year's case stretched for 12 months before the June 8 ballot.