Britain's electoral commission 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 with Aggregate IQ, a company that used social media data to target voters, under a common plan with Vote Leave.

It means Vote Leave exceeded its legal spending limit of £7mn by almost £500,000.

"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," said Bob Posner, the commission's director of political finance and regulation.

Vote Leave was fined £61,000 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.

Mr Posner said: "Vote Leave has resisted our investigation from the start, including contesting our right as the statutory regulator to open the investigation.

"It has refused to cooperate, refused our requests to put forward a representative for interview, and forced us to use our legal powers to compel it to provide evidence.

"Nevertheless, the evidence we have found is clear and substantial, and can now be seen in our report."

Vote Leave said there was no evidence of any wrongdoing.

"The Electoral Commission's report contains a number of false accusations and incorrect assertions that are wholly inaccurate and do not stand up to scrutiny," a spokesman for Vote Leave said in a statement.

He accused the commission, which said Vote Leave has resisted its investigation and committed serious breaches of the law, of being motivated by a political agenda and said the group was confident that the findings would be overturned.

Additional reporting PA