* Leave.EU fined 70,000 pounds for breaking election law
* Leave.EU overspent in referendum, says electoral commission
* Founder Arron Banks says commission is partisan
* Leave.EU did not receive donations from Cambridge Analytica (Adds details, quotes)
By Alistair Smout and Guy Faulconbridge
LONDON, May 11 (Reuters) - Britain's Electoral Commission imposed a record-matching 70,000 pound fine on Friday on one of the main groups that campaigned for Brexit, and said police might have to investigate possible criminal offences because of unreported campaign spending.
The Leave.EU campaign said it would challenge the finding that it had breached a spending limit by failing to declare at least 77,380 pounds ($105,000). It said the alleged breaches were minor and the report proved there was no "big conspiracy" around the Brexit vote.
The documented breaches of electoral law could still fuel demands from opponents of Brexit for a re-run of the 2016 referendum. The commission said it was enforcing the rules without suggesting the breach had altered the result.
"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."
Asked on BBC radio if the commission was saying the breach was serious enough to have impacted the result, its chief executive, Claire Bassett, said "No", but she added that the rules still needed to be enforced.
The commission said it suspected criminal offences may have been committed, and the person responsible, Leave.EU CEO Liz Bilney, had been referred to the police.
In the June 23, 2016 referendum, 17.4 million voters, or 51.9 percent, backed leaving the EU while 16.1 million voters, or 48.1 percent, backed staying.
Arron Banks, the founder of Leave.EU who was pictured with Donald Trump and leading Brexiteer Nigel Farage outside a gilded elevator soon after the 2016 U.S. presidential election, cast doubt on the commission's impartiality.
"The Electoral Commission is a 'Blairite Swamp Creation' packed full of establishment 'Remoaners'," Banks said, using an epithet that Brexit supporters have coined for those, such as former Prime Minister Tony Blair, who have continued to campaign to remain in the EU despite the referendum.
"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. "What a shambles. We will see them in court."
The commission said it had found no evidence that Leave.EU received donations or paid-for services from the firm 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 referendum. Lawmakers have called for investigations into any role the firm may have had in the Brexit campaign.
The commission said Leave.EU failed to include services it received from U.S. campaign strategy firm Goddard Gunster in a spending return, and inaccurately reported three loans. Leave.EU exceeded the spending limit for non-party registered campaigners by at least 10 percent, it said.
The fine is the joint biggest handed down by the Electoral Commission, matching a fine given to Prime Minister Theresa May's Conservative party for inaccurately reporting spending in elections in 2014 and 2015.
Leave.EU said the total alleged overspending represented less than 0.1 percent of overall spending on the campaign, and the charges against it fell well short of what the commission thought it would find when it started its investigation.
"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.
$1 = 0.7398 pounds Editing by Richard Balmforth and Peter Graff