LONDON (AP) — The official group that campaigned to take Britain out of the European Union in the 2016 referendum has been fined and referred to police for breaking electoral law, Britain's electoral watchdog said Tuesday.

The Electoral Commission said the "Vote Leave" group, backed by senior politicians including former foreign secretary Boris Johnson, failed to declare 675,000 pounds ($894,000) it spent with Canadian data firm Aggregate IQ. The undeclared spending meant the campaign group exceeded the 7-million-pound legal spending limit by almost 500,000 pounds.

The commission said it found significant evidence that Vote Leave did this by funneling cash to a small, unregistered pro-Brexit youth group, BeLeave.

"Vote Leave has resisted our investigation from the start. ... 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," said Bob Posner, who oversees political finance at the commission.

"Nevertheless, the evidence we have found is clear and substantial."

Those who wanted Britain to leave the EU narrowly won the June 2016 referendum, with 52 percent of voters supporting Brexit.

The election fraud probe became entangled with inquiries into Facebook's use of private data, because Aggregate IQ had links with Cambridge Analytica, the British consultancy accused of using data from tens of millions of Facebook accounts to help Donald Trump win the 2016 U.S. election.

Whistleblowers had alleged that Brexit campaigners paid Aggregate IQ to send targeted ads and that their actions may have unfairly influenced the referendum's outcome.

Vote Leave was fined 61,000 pounds. BeLeave founder Darren Grimes, a student at the time, was fined 20,000 pounds. The commission said Grimes and an official from Vote Leave have both been referred to police "in relation to false declarations of campaign spending."

Grimes and Vote Leave deny wrongdoing.

Another pro-Brexit organization, Leave.EU, was fined 70,000 pounds in May for overspending and filing inaccurate records.