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 previous year. Here's everything you need to know.
Vote Leave finance director Antonia Flockton told a Commons select committee past year: "There is a question as to whether they were independent campaigns acting independently". Labour MP Ben Bradshaw has raised questions over Banks's ties to "dark money" and Russian involvement in the Brexit campaign.
Banks, who has declared his admiration for Russian president Vladimir Putin and held a lengthy lunch with Russia's United Kingdom ambassador, says allegations that Russians gave him political funding are "complete bollocks".
A second investigation is looking into whether Leave.EU illegally used the services of Cambridge Analytica, a data analytics firm largely owned by American billionaire Robert Mercer, who backed United States president Trump, and until recently owned a stake in far-right news organization Breitbart.
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Investigators will examine whether Vote Leave - backed by Tory Cabinet ministers Boris Johnson and Michael Gove - filed returns correctly, says The Guardian.
Cambridge Analytica's involvement with Leave.EU is unclear.
On the first investigation, which is looking at links between Leave.EU, Cambridge Analytica and advocacy firm Goddard Gunster, he said it was yet to be resolved despite going on for "200 plus days". Campaigns can then use that data to work out who and how to target.
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.
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The Commission said it had "reasonable grounds to suspect an offence may have been committed".
The big question is whether Vote Leave told Grimes to do that.
Darren Grimes set up BeLeave, a group representing young people in the campaign for a leave vote.
Last week the commission's head of regulation Louise Edwards said in a letter: "Investigations can take further time where the Commission needs to go back to organisations to ensure that full disclosure of requested material has been provided".
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Veterans for Britain also received a donation worth £100,000 from Vote Leave in the run-up to the June 23 vote.
The Brexit campaigner says he may seek a judicial review if the Electoral Commission doesn't reveal who handed it information. "This has all been declared as required by the Electoral Commission". All other campaigners were capped at £700,000 each. If found guilty, it could impose a fine of up to £20,000 for each offence.
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