Registered campaigners like Mr Grimes and Veterans for Britain were permitted to spend £700,000.
The fashion student was registered as an official participant in the referendum while Vote Leave was the lead campaigner for Out.
The campaign, run by political strategist Matthew Elliott and former special adviser Dominic Cummings, will be investigated alongside Veterans for Britain and student activist Darren Grimes, now the deputy editor of the Brexit Central website, where Elliott is now editor-at-large.
THE OFFICIAL Brexit campaign group Vote Leave is under investigation over its referendum spending and whether it exceeded limits and entered an incorrect return.
Vote Leave gave £625,000 to Darren Grimes shortly before the European Union referendum.
Explaining the decision, it said: "New information has come to light which, when considered alongside the information obtained previously, has given the Commission reasonable grounds to suspect an offence may have been committed".
Its statement continued: "There is significant public interest in being satisfied that the facts are known about Vote Leave's spending on the campaign, particularly as it was a lead campaigner with a greater spending limit than any other campaigners on the "leave" side", said a spokesperson for the Electoral Commission.
In February, the Electoral Commission told BuzzFeed News that they could not prove any break of the rules "beyond reasonable doubt", after Labour MP Stephen Kinnock wrote to demand an urgent investigation into the arrangement.
His intervention came after the commission said the investigation into Leave.EU was being delayed because the Brexit campaign group had failed to hand over information.
A previous probe found nothing, but has been re-opened in light of "new information".
The Electoral Commission announced earlier this month it was investigating Arron Banks and Leave.EU over whether they received illegal donations.
In its announcement on Monday, the commission said it was investigating whether Grimes and Veterans for Britain had delivered incorrect spending returns in relation to the Vote Leave donations.
It is looking into the alleged undeclared provision of services to Leave.EU by data firm Cambridge Analytica.
"In relation to our first investigation in respect of Leave.EU, the commission will shortly set out to Leave.EU areas where it appears material has not been provided, notably relating to documents comprising the services provided by Goddard Gunster in late 2015/early 2016".