One of the main Brexit campaign groups in Britain's 2016 EU referendum has been fined STG70,000 ($A126,330) for incorrectly reporting what it spent and exceeding a limit on expenditure, the Electoral Commission says.
The commission said on Friday that Leave.EU incorrectly reported what it spent at the EU referendum, failing to include at least STG77,380 in its spending return and thus exceeding a spending limit.
Such is the seriousness of the offences that the commission said it suspected criminal offences may have been committed by the person responsible and referred the case to the police.
"These are serious offences," said Bob Posner, the Electoral Commission's director of political finance and regulation.
"Leave.EU exceeded its spending limit and failed to declare its funding and its spending correctly," Posner said.
In the June 23, 2016 referendum, 17.4 million votes, or 51.9 per cent of votes cast, backed leaving the EU while 16.1 million votes, or 48.1 per cent of votes cast, backed staying.
Leave.EU said the total alleged overspend represented less than 0.1 per cent of the total campaign finance spend and was a risible attempt to portray the campaign as breaking the rules.
Arron Banks, the founder of Leave.EU, cast doubt on the Electoral Commission's impartiality, saying it was packed full of opponents of Brexit, who are sometimes known as "Remoaners".
"The Electoral Commission is a 'Blairite Swamp Creation' packed full of establishment 'Remoaners'," Banks said.
"We view the Electoral Commission announcement as a politically motivated attack on Brexit and the 17.4 million people who defied the establishment to vote for an independent Britain," he said.
The commission said Leave.EU failed to include services it received from US campaign strategy firm Goddard Gunster in a spending return. The commission also found Leave.EU inaccurately reported three loans it had received.
But the voting regulator said it found no evidence that Leave.EU received donations or paid-for services from Cambridge Analytica, a political consultancy at the centre of a storm over how Facebook data was used in political campaigns.
Leave.EU and Cambridge Analytica had previously denied working together on the Brexit campaign.
"The Electoral Commission went big game fishing and found a few 'aged' dead sardines on the beach. So much for the big conspiracy!" Banks said.
"What a shambles, we will see them in court."
