The grassroots group Momentum has been fined £16,700 for “multiple breaches of electoral law”, the Electoral Commission has said.
The fines include £12,150 for an inaccurate 2017 general election spending return, the highest fine levied on a non-party campaigner, the Electoral Commission said.
Momentum was also fined for failures to report donations accepted as a “members association” outside of an election period.
Louise Edwards, the commission’s regulation director, said: “Non-party campaigners are essential for a healthy democracy. But just as crucial is that after a poll, voters can see complete and accurate spending data.
“The fines that we have levied reflect Momentum’s repeated revisions to their spending return, poor record keeping and failure to follow advice given by the commission prior to the election.”
Ms Edwards added: “Momentum is unlike most non-party campaigners in that political campaigning is its full-time work, so it is particularly disappointing that they have failed to meet the law’s requirements.”