In a report published on Tuesday, the Electoral Commission fined Vote Leave 61,000 British pounds ($81,000) for exceeding the legal spending limit of 7m British pounds (about $9.2m) by almost 500,000 British pounds ($662,000), The organisation broke the rules by working with another pro-Brexit campaign group, BeLeave, without declaring it, the report said.
In the referendum, the official pro- and anti-Brexit campaign groups, designated by the Electoral Commission, had spending limits of £7 million (US$9.3 million, €7.9 million).
The drama around Brexit has managed to stay at a fever pitch for two years now, and there doesn't seem to be any chance of it abating any time soon, especially with revelations that one of the major "leave" groups likely broke electoral rules.
Vote Leave exceeded its legal spending limit of £7m by nearly £500,000, the watchdog found.
Darren Grimes, the founder of BeLeave, has also been fined £20,000.
While opponents of Brexit have claimed the campaign to leave the European Union cheated in the referendum, the Electoral Commission's chief executive Claire Bassett told BBC radio that its investigation was focused only on campaign spending violations.
Separate campaign groups with their own spending limits are not permitted to co-ordinate strategy.
The move follows a probe into campaign spending.
Giant LeBron banner to be removed again in Cleveland
We wish his kids, his wife Savannah, his mother Gloria, and LeBron himself nothing but the best in the years and decades ahead. There was a time when Gilbert refused to consider the possibility. "LeBron, you came home and delivered the ultimate goal.
"Vote Leave has provided evidence to the Electoral Commission proving there was no wrongdoing".
During an urgent question in the House of Commons on the report, Labour said the cabinet secretary, Jeremy Heywood, should investigate whether ministers and ex-ministers such as Michael Gove and Boris Johnson, who were key figures in Vote Leave, had broken the ministerial code.
"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", he said.
According to a statement on the Commission's website, their investigation "found significant evidence of joint working between the lead campaigner, Vote Leave and another campaign group BeLeave".
He told BBC Scotland that he did not want to prejudge the allegations against the national Vote Leave organisation, which he said was likely to be decided in court.
"Nevertheless, the evidence we have found is clear and substantial", Posner said.
A spokesman for Vote Leave said the report contained a "number of false accusations and incorrect assertions that are wholly inaccurate and do not stand up to scrutiny".
"All this suggests that the supposedly impartial commission is motivated by a political agenda rather than uncovering the facts".
The Electoral Commission's findings come as Prime Minister Theresa May's Conservative party remains starkly divided over if and how the United Kingdom should split from the European Union. "Consequences must follow. We can not have confidence that this referendum was secure, and it should be re-run", she said.