The grassroots pro-Jeremy Corbyn campaign group Momentum has been fined £16,700 for “multiple breaches of electoral law”, the Electoral Commission has said.
The watchdog said the fines were related to activity during the 2017 general election.
Momentum hit back at the “comic” level of detail it was required to provide.
It includes a penalty of £12,150 for not submitting a “complete and accurate spending return”, the highest fine ever levied on a non-party campaigner.
Louise Edwards, director of regulation at the Electoral Commission, said voters had a right to 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,” she said.
“Non-party campaigners that seek to persuade people to vote a certain way rightly have legal obligations; it is incumbent on them to invest properly in having the right processes and staff to meet their obligations.
“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.”
Momentum helped propel Corbyn to the Labour leadership and campaigned vigorously on the party’s behalf at the 2017 election.
In a statement the organisation said: “The level of detail required under the law was often comic. In one instance, the Electoral Commission queried the purchase of a pizza, wanting to know what percentage of the pizza was eaten by staff members and what percentage by volunteers.
“These gagging laws, introduced by the Conservatives and opposed at the time by hundreds of campaign groups, are deliberately designed to leave big money unregulated whilst throttling volunteer led, campaigning organisations with burdensome regulation which in some cases is almost impossible to follow.”