Momentum, the Jeremy Corbyn-supporting Labour grassroots movement, has been fined £16,700 after what the Electoral Commission described as “multiple” breaches of electoral law related to spending returns and failures to report donations.

The total included a fine of £12,150 for submitting a spending return for the 2017 general election that was not complete and accurate, the elections watchdog said. This was the highest such fine levied on a non-party campaign, it added.

The commission said: “This is the first time that Momentum has been investigated. In order to meet their legal reporting obligations in future, they must ensure they have the right staff and processes in place.”

As a registered non-party campaigner, with the spending limits entailed in electoral law, Momentum submitted a spending return in September 2017. The commission launched an investigation in November that year after “a number of issues were identified with the return”.

The results of the investigation found the group omitted almost £23,000 of reportable donations from a post-poll donation report, and failed to provide all the necessary declarations and invoices.

The organisation also received two smaller fines for delays in reporting donations to it from the Transport Salaried Staffs’ Association (TSSA) union, which should have been done within 30 days.

The commission said one £10,000 donation was received in July 2016, but not reported until January this year. Another sum of £8,000 was received in May 2017 but not reported until July 2018.

Louise Edwards, the head of regulation at the Electoral Commission, 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.”

In a statement, Momentum spokeswoman Laura Parker said the group had been initially investigated over allegations it spent more during the 2017 election than the regulated spending limit of £37,920, but that the commission found this was not the case.

“It is understandable that the Electoral Commission would investigate Momentum – we managed to do a lot with very little. Our general election campaign was powered by tens of thousands of volunteers who helped out in our office, ran training sessions and knocked on doors in marginal constituencies across the country,” Parker said.

On the errors that were made, she said: “This isn’t surprising for a new organisation, which at the time was less than two years old and had 25,000 members and 150 local groups. The Conservatives likely employ more lawyers than Momentum have staff, and even getting close to fully complying with these complex regulations for a volunteer-led, social movement organisation is a herculean task.”

But, Parker added, the level of the fines was “disproportionate”. She said: Not only did Momentum cooperate fully with the Electoral Commission, but these offences are incredibly minor when compared with other political organisations.”