Brexit bankroller Arron Banks’s close relationship with the controversial data firm Cambridge Analytica – and the key role played by former Trump chief strategist Steve Bannon in the early days of Banks’s Brexit campaign – have been laid bare in explosive new emails obtained by openDemocracy.

Banks, who is currently under investigation by the National Crime Agency over the sources of his £8.4m Brexit donation, told Parliament in June that he had “initial discussions” with the controversial data firm Cambridge Analytica but “did not take up their services”.

Emails and documents obtained by openDemocracy show that:

Far-right guru Steve Bannon, Trump’s former campaign chief, personally introduced Cambridge Analytica to Banks’s Brexit campaign, which is now under criminal investigation.

Banks asked Cambridge Analytica “to come up with a strategy” for his Leave.EU campaign to raise funds in the United States months before the Brexit vote. Donations from overseas are not permitted under British law.

Cambridge Analytica was given access to personal information about British voters from Banks’s Leave.EU campaign, including access to social media accounts and call centre data. Banks has previously claimed to Parliament that "the only data that was ever sent to Cambridge Analytica was from UKIP”.

Cambridge Analytica discussed working with Bank’s Eldon Insurance company. Leave.EU had previously said “we never involved the insurance company and Cambridge Analytica ever.”

The revelations come as Banks is poised to play a key role in pushing for a hard Brexit. On Twitter, Banks’s Leave.EU campaign has declared “May must go!” and has encouraged supporters to join the Conservatives to force the prime minister out of office.

A number of the emails obtained by openDemocracy are expected to be published as evidence by Parliament's inquiry into fake news this coming week.

Commenting on openDemocracy's revelations, Damian Collins, chair of the inquiry said: "This is more evidence that Arron Banks misled the select committee when he gave evidence to Parliament.

"Here we can glimpse how these secret connections were being planned and discussed; the incubation of a political virus involving key people, data, money and consultancies like Cambridge Analytica.

He added: "Why was (Banks) seeking support from Cambridge Analytica with fundraising in America if all the money for Leave.EU came from his own resources? These emails should form part of the NCA investigation into Arron Banks’s finances.

"The emails also show that contact between key Trump aides like Steve Bannon, and men like Arron Banks, was not just passing, but that they were working together through and involving common businesses, like Cambridge Analytica."

Dr Emma L Briant, who has submitted similar evidence to the parliamentary committee, said: "Leave.EU funder Arron Banks denies claims that he may have received Russian money for Brexit. This new evidence shows that Banks was seeking foreign funding for Brexit from the very beginning."

"The emails also reveal new evidence of what constituted Banks’s ‘two stage’ plans for Leave.EU involving Cambridge Analytica. If he did not need their services, why did he go to them? The Electoral Commission must re-open their investigation into Leave.EU, as this evidence reignites questions of whether undeclared services were part of the campaign."

Cambridge Analytica, which initially came to prominence for its role in the 2016 Trump campaign, was wound up after the Observer newspaper revealed that it had harvested data on tens of millions of Facebook users and engaged in ‘black ops’ political campaigns around the world.

The National Crime Agency announced that it was investigating Banks earlier this month after the UK elections watchdog reported concerns that the £8.4m Banks spent on the Brexit campaign – the biggest single donation in British political history – came from impermissible sources outside the UK. Banks has denied this.

The Steve Bannon connection

openDemocracy recently revealed that Banks misled Parliament by saying that Leave.EU and his Eldon insurance business were separate businesses. We also showed that his insurance staff had access to personal information on tens of millions of voters of British voters gathered from electoral rolls from across the UK. Banks has denied misusing any data.

However the new emails leaked to openDemocracy provide a much clearer picture of the relationship between Arron Banks and Cambridge Analytica than has previously been known.

In August 2016, just a few weeks after the Brexit vote, Steve Bannon, then Donald Trump’s campaign chief, invited Banks to a political fundraiser in Mississippi. This was not Banks’s first engagement with Bannon. Almost a year earlier, it was Bannon who brought Cambridge Analytica and Banks’s Brexit campaign together.

Just a month after Leave.EU was founded, communications seen by openDemocracy indicate that the former Breitbart chief introduced Banks to Alexander Nix, Cambridge Analytica’s onetime CEO, and arranged a follow-up phonecall between Banks and Cambridge Analytica. Bannon and Nix were together in the US on this call, along with other staff from the data analytics firm.

On 24 October, the day after the scheduled meeting with Cambridge Analytica, Banks, in an email sent to some of his closest associates, said that he “would like CA to come up with a strategy for fund raising in the States and engaging companies and special interest groups that might be affected by TTIP (Transatlantic Trade and Investment Partnership)”.

Steve Bannon was among the email’s recipients.