In a statement on its website, the campaign finance watchdog said it has begun an inquiry into whether Vote Leave broke the £7 million ($9.3 million) spending limit for referendums, adding that new information has come to light which means it has "reasonable grounds to suspect an offence may have been committed".
It is possible that during the course of the investigation, the Commission will identify potential contraventions and/or offences under PPERA other than those set out above.
Vote Leave - led by Boris Johnson and Michael Gove - gave Mr Grimes the money to support his BeLeave campaign in the run-in to the June 23 referendum a year ago.
Vote Leave reported spending totalling nearly £6.8 million on the referendum, bringing it close to the £7 million limit for the designated lead campaign.
"Legitimate questions over the funding provided to campaigners risks causing harm to voters" confidence in the referendum and it is therefore right that we investigate'.
The watchdog is also separately looking into whether Mr Banks was the "true source" of three loans worth £6 million on non-commercial terms to Leave.EU, and whether Better For The Country Limited (BFTCL) - a company that lists him as a director - was acting as an "agent" when it donated £2.3 million to five registered campaigners.
The Electoral Commission has launched a new investigation into spending at the European Union referendum by Vote Leave, Darren Grimes and Veterans for Britain.
The inquiry is understood to be related to two donations totalling £725,000 that were given to Darren Grimes, a 23-year-old fashion design student and Brexit supporter, and a group called Veterans for Britain in the final days of the campaign.
The Electoral Commission is already conducting investigations into other parts of the Brexit campaign and the new inquest re-opens an early issue.
Grimes and Veterans for Britain will be investigated as to whether he delivered an incorrect spending return in relation to a donation they received from Vote Leave and related campaign spending.
The commission is looking into the alleged undeclared provision of services to Leave.EU by data firm Cambridge Analytica.
Jo Maugham QC, of the Good Law Project, said: 'We are 18 months after the referendum vote. "This has all been declared as required by the Electoral Commission". "And only in response to legal action", he said.
At the time, no further action took place.