The Vote Leave group - which campaigned for a Leave vote in the Brexit referendum - has been referred to police in the UK for breaking electoral law.
The British Electoral Commission has also fined the campaign following an investigation into its spending.
Vote Leave was the main group that successfully campaigned for a Leave Vote in June 2016, and was fronted by prominent campaigners such as Boris Johnson and Michael Gove.
The commission has now found evidence of 'joint working' between Vote Leave and another unregistered youth group called BeLeave.
According to investigators, more than £675,000 (€760,000) paid by BeLeave to Canadian data analytics firm Aggregate IQ should instead have been declared by Vote Leave.
As a result, the commission says that Vote Leave group exceeded its legal spending limit of £7 million (€7.88 million) by just under £500,000 (€563,000).
Vote Leave also returned an "incomplete and inaccurate" report that saw more than £230,000 (€259,000) in spending reported incorrectly, along with missing invoices for almost £13,000 (€14,600) worth of transactions.
BeLeave founder Darren Grimes is said to have committed two offences, and has been fined £20,000 (€22,500).
Mr Grime's and Vote Leave's David Halsall have both been referred to police in London "in relation to false declarations of campaign spending".
"Refused to cooperate"
The Electoral Commission's Bob Posner explained: "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."
The commission insisted it conducted a "thorough and fair investigation".
In the wake of the investigation, senior Conservative MP Sarah Wollaston called for a second Brexit referendum, arguing: "Brexit was not only sold on deliberate lies & false promises, but also by breaking electoral law."
