An individual who volunteered for a Brexit campaign effort said the organization broke the law, and in turn, is arguing the referendum vote "wasn't legitimate," according to the United Kingdom's Channel 4 News.

Shahmir Sanni, a former volunteer with the Vote Leave group, told Channel 4 that Vote Leave's donation of 625,000-pound to another Brexit campaign group, BeLeave, meant it violated spending rules because the groups worked together.

"In effect they used BeLeave to overspend and not just by a small amount," Sanni told the TV channel. "They say that it wasn't coordinated but it was. So the idea that the campaign was legitimate is false."

Dominic Cummings, a former Vote Leave director, denied his group broke any rules, saying it had received permission from the Electoral Commission to make donations in the run-up to the referendum.

In a post on his blog, Cummings also said the allegations were part of an attempt by supporters of Britain's continued EU membership to overturn the referendum decision to leave.

Nearly two years after the vote, British public opinion remains split on the merits of leaving the EU, with each side accusing the other of having misled voters. The government says Britain will leave on schedule in March 2019 without a another referendum.