Democracy watchdog the Electoral Commission has opened an investigation into EU referendum spending by the Vote Leave campaign.

As well as looking into whether the main pro-Brexit campaign exceeded spending limits and entered an incorrect return, the Commission will also investigate donations made by Vote Leave to student Darren Grimes and Veterans for Britain.

The announcement of the inquiry came as anti-EU millionaire Arron Banks, chair of the separate Leave.EU organisation, threatened legal action against the Commission over its investigation into his Brexit campaign.

Electoral Commission figures show that Mr Grimes received donations totalling £625,000 from Vote Leave, which – along with £50,000 from an individual donor – allowed the 23-year-old to spend £675,000 on the BeLeave social media campaign to encourage young people to vote to quit the EU in last year’s referendum.

Veterans for Britain also received a donation worth £100,000 from Vote Leave in the run-up to the June 23 2016 vote.

Vote Leave reported spending totalling almost £6.8 million on the referendum, bringing it close to the £7 million limit for the designated lead campaign. Registered campaigners like Mr Grimes and Veterans for Britain were permitted to spend £700,000.

Announcing its inquiry into Vote Leave, the commission said: “The opening of this investigation follows a review of previous assessments that the Electoral Commission conducted in February and March 2017 which, at the time, resulted in no further action being taken.

“Since that time, 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.”

Vote Remain and Vote Leave signs on a lamp post(Danny Lawson/PA)

Meanwhile, the commission was facing a threat of legal action from Mr Banks unless it reveals the sources behind an investigation into the funding of his Brexit campaign, Leave.EU.

The insurance tycoon said he would seek a judicial review of the decision to launch the probe unless the Electoral Commission gave details of its sources within 21 days.

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.

The commission is looking into the alleged undeclared provision of services to Leave.EU by data firm Cambridge Analytica.

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.

Mr Banks said the investigation into the links to Cambridge Analytica and advocacy firm Goddard Gunster had not been resolved after “200 plus days”.

Arron BanksArron Banks (Jonathan Brady/PA)

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.

“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.”

But in his response Mr Banks said: “At the outset of the investigation we sent you three lever arch folders full of the relevant information and the last we heard from you was in July.

“There have been no requests for further information or chasers.”

He claimed the commission has “sat on” the investigation because it did not have a “shred of evidence” but it was “too embarrassing for you to conclude”.

On the second investigation into whether “dark Russian money” or overseas donations had been made to Leave.EU, he said: “Unless you set out in the next 21 days the source of these allegations and why you find them credible, we intend to seek a judicial review of the decision to investigate our campaign and the political reasons why.”

An Electoral Commission spokesman said the watchdog had seen Mr Banks’ open letter “and will be responding in due course”.