Britain’s electoral commission says it has fined the officially designated pro-Brexit campaign group for breaching spending rules in the 2016 referendum and referred it to the police.

The commission said Vote Leave worked with another campaign group, BeLeave, which spent 675,000 pounds ($A1.2 million) with Aggregate IQ, a company that used social media data to target voters, under a common plan with Vote Leave.

“We found substantial evidence that the two groups worked to a common plan, did not declare their joint working and did not adhere to the legal spending limits,” Bob Posner, the commission’s director of political finance and regulation.

Mr Posner told the BBC that Vote Leave had resisted the investigation from the start and had refused to co-operate.

“Nevertheless, the evidence we have found is clear and substantial,” Mr Posner said.

“These are serious breaches of the laws put in place by Parliament to ensure fairness and transparency at elections and referendums.”

Shahmir Sanni and Christopher Wylie explain their concerns in March. Photo: Getty

Vote Leave was fined 61,000 pounds ($A108,000) and the Electoral Commission referred David Halsall, the responsible person for Vote Leave, and Darren Grimes, the founder of the BeLeave campaign group, to the police for false declarations of campaign spending.

The action was taken after former Vote Leave volunteers turned whistleblowers Shahmir Sanni and Christopher Wylie alleged in March that Vote Leave may have broken referendum spending rules and then tried to destroy evidence.

Aggregate IQ has links with Cambridge Analytica.

-AAP