The Vote Leave campaign was the official face of the 2016 effort to persuade Britain to leave the European Union — an effort that succeeded. "Leave" won nearly 52 percent of the vote in the contentious Brexit referendum that year, and Britain is slated to leave the E.U. next March.

But on Tuesday, the Electoral Commission, an independent watchdog group, said it had found "significant evidence" that Vote Leave broke British electoral law. The commission said the group did not properly declare the extent of its coordination with BeLeave, a pro-Brexit youth group; by spending about $887,000 through BeLeave, Vote Leave thus allegedly overspent its legal $9.2 million limit. That money reportedly went to Aggregate IQ, a Canadian digital-marketing firm.

The commission's findings came with fines: about $26,270 for Darren Grimes, the founder of BeLeave; and $80,000 for Vote Leave. Both Grimes and Vote Leave campaigner David Halsall have also been referred to the police for allegedly filing false reports about their campaign spending.

In a statement posted on Twitter, Grimes said he is "shocked and disappointed by the Electoral Commission and their behaviour."

"Politicians say they want young people to engage with politics," he wrote. "I was 22 when I got involved in a referendum I felt passionately about. I did nothing wrong."

Vote Leave said in a statement that the commission's report "contains a number of false accusations and incorrect assertions that are wholly inaccurate and do not stand up to scrutiny."

But Bob Posner, director of political finance at the Electoral Commission, said his organization had "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."

"These are serious breaches of the laws put in place by Parliament to ensure fairness and transparency at elections and referendums," he said in a statement.

Did overspending affect the outcome of the Brexit vote? That will be difficult to determine. But Shahmir Sanni, a Vote Leave campaigner-turned-whistle-blower told the BBC that the additional money "can make all the difference."

As The Washington Post's William Booth and Karla Adam reported in March, "Sanni said he and Grimes were based in the Vote Leave headquarters; were advised by Vote Leave staffers, including [Prime Minister Theresa] May’s now-senior adviser; and relied on Vote Leave’s attorney, who helped them incorporate the BeLeave group."

Lawmaker Chuka Umunna told Parliament on Tuesday that "we cannot say with confidence that this foul play did not impact on the result.”

Tuesday's report landed at a sensitive time for the prime minister. Last week her cabinet was rocked by resignations, including that of David Davis, the minister who was supposed to help negotiate Brexit.

The day after Davis resigned, Foreign Secretary Boris Johnson also stepped down, criticizing May's Brexit strategy and saying "[the Brexit] dream is dying, suffocated by needless self-doubt." Johnson was a key Vote Leave campaigner, and on Tuesday, members of the Labour Party called for investigations into both him and Environment Minister Michael Gove, who also backed the Vote Leave campaign.

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