Far-right group Britain First has been fined £44,200 by the Electoral Commission for “multiple breaches” of electoral law.
The elections watchdog said the group had filed to keep accurate financial records, provide donation reports or properly audit its accounts.
It added that the breaches occurred in 2016, before the group was de-registered as a political party.
“Basic requirements” of elections law had not been met, it said.
Britain First, an offshoot of the BNP, campaigned for a near-total halt to immigration.
It fared poorly in the three elections where it stood as a party, including in the 2016 London mayor election where it won 1.2% of the votes, and was de-registered by the commission in November 2017, after it failed to renew its registration on time.
But the watchdog said the group remained responsible for its actions during its time as a party.
In a statement, the watchdog said after an investigation it had decided to impose a fine of £11,000 on the group for failing to keep accurate financial records of its transactions in 2016.
There was a further £7,700 fine for failing to provide quarterly donations reports and £5,500 for failing to properly audit its accounts during the same year.
The watchdog added that during 2016, Britain First had received £200,000 worth of undeclared donations.
The biggest fine of £20,000 – the maximum permitted under the rules – was for failing to provide information to the commission when requested.
‘Lack of transparency’
Louise Edwards, the watchdog’s director of regulation, said registered parties were under an obligation to provide “full, accurate data” on their finances.
“Britain First’s inability to meet certain basic requirements led to a disappointing lack of transparency into the party’s finances”, she added.
“The party’s failure to comply with a statutory notice is a serious offence and shows a disregard for the law.
“Throughout our investigation, we saw little to suggest that Britain First appreciated that if it wants to contest elections, it must meet its legal responsibilities, which is why we have imposed the penalties that we have.”
In March last year, the group’s page on Facebook was removed by the social media firm, which said it had repeatedly violated its community standards.