The lead pro-Brexit campaign group Vote Leave has been fined £61,000 and referred to police by the UK’s elections watchdog for breaking electoral laws on campaign spending.

The Electoral Commission today published the conclusions of its investigation into the spending of Vote Leave and other campaigners, finding that Vote Leave and Darren Grimes broke electoral law.

The investigation found significant evidence of joint working between Vote Leave and another campaign group called BeLeave.

Evidence shows that BeLeave spent more than £675,000 with AggregateIQ under a common plan with Vote Leave. This spending should have been declared by Vote Leave, which means Vote Leave exceeded its legal spending limit of £7 million by almost £500,000.

Vote Leave also returned an incomplete and inaccurate spending report, with nearly £234,501 reported incorrectly, and invoices missing for £12,849.99 of spending.

Mr Grimes, founder of BeLeave, was found to have committed two offences and has been fined £20,000. He spent more than £675,000 on behalf of BeLeave, a non-registered campaigner with a spending limit of £10,000. He wrongly reported that same spending as his own.

The commission has now referred David Halsall, the responsible person for Vote Leave, and Mr Grimes to the Metropolitan Police in relation to false declarations of campaign spending.

It has also shared its investigation files with the Metropolitan Police in relation to whether any persons have committed related offences which lie outside the Commission’s regulatory remit.

Bob Posner, the Electoral Commission’s director of political finance and regulation & legal counsel, said: “These are serious breaches of the laws put in place by Parliament to ensure fairness and transparency at elections and referendums. Our findings relate primarily to the organisation which put itself forward as fit to be the designated campaigner for the ‘leave’ outcome.”

He added that Vote Leave had “resisted our investigation from the start” and forced the watchdog “to use our legal powers to compel it to provide evidence”. He added: “Nevertheless, the evidence we have found is clear and substantial, and can now be seen in our report.”

The investigation also found that the campaign group Veterans for Britain inaccurately reported a donation it received from Vote Leave. It has been fined £250. There was no evidence that Veterans for Britain campaigned under a common plan with Vote Leave.

In total, the levels of fines are £61,000 for Vote Leave, £20,000 for Mr Grimes and £250 for Veterans for Britain.

The commission noted that it was constrained by the maximum individual fine limit of £20,000, which it considers to be “inadequate for serious offences of electoral or referendum law”.