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 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 organisation 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.
Did overspending actually affect the outcome of the Brexit vote? That will be difficult to determine. But Shahmir Sanni, a Vote Leave campaigner-turned-whistleblower told the BBC that the additional money "can make all the difference."
As the Washington Post 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 lawyer, who helped them incorporate the BeLeave group."
MP Chuka Umunna told Parliament that "we cannot say with confidence that this foul play did not impact on the result."
The 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, criticising May's Brexit strategy and saying "[the Brexit] dream is dying, suffocated by needless self-doubt."
Johnson was a key Vote Leave campaigner, and today, members of the Labour Party called for investigations into both him and Environment Minister Michael Gove, who also backed the Vote Leave campaign.
In Parliament, May faced down a rebellion over Brexit, with MPs rejecting a key amendment on a trade bill.
Pro-EU MPs — from both May's Conservatives and the opposition Labour Party — had tried to force through a measure to have Britain to join a European customs union should there be no trade agreement with Brussels by January. But the pro-EU MPs lost today by six votes — 307 to 301.
The bill gives the Government the power to set up new international trade relationships after Britain leaves the EU next March.
The win is the second for May in two days, as her government avoided defeat in Parliament on a separate bill.
- additional reporting AP
