Arron Banks, who funded Nigel Farage’s Leave.EU campaign, talked about using private investigators to dig up dirt on journalists looking into his business activities, evidence seen by the Observer suggests.

Last week, the Observer published evidence that suggested Leave.EU co-ordinated its social media messaging with the Kremlin, including re-tweeting attacks against journalists and MPs by the Russian embassy.

But shocking new evidence includes emails from the same period in which Banks and Andy Wigmore, Leave.EU’s press spokesman and Banks’s business partner, appear to discuss hunting for “personal stuff” to use against Ian Katz, who was editor of the BBC’s Newsnight at the time.

On 24 October, Newsnight broadcast a report on Banks by the journalist John Sweeney. The question of his finances and the source of his donations to the Leave.EU campaign has been a subject of speculation since the referendum and is being investigated by the Electoral Commission.

Seven days before the broadcast, after the BBC submitted a set of questions to Banks, evidence seen suggests an employee emailed Banks and Wigmore with research on Katz’s background – and that Wigmore replied: “We need personal stuff like girlfriends, if he’s in debt all the stuff a private investigator can find. That’s how we should retaliate.”

The documents suggest Banks responded: “Agreed. Have a chat with Patrick and let’s get motoring on.” 

When asked about the allegations, Wigmore told the Observer: “You will be aware that we have reported you, the Guardian, the Observer, Peter Jukes Bi-Line Media [sic] and Chris Wylie to the police under the Computer Misuse Act 1990.

“The police have also been given evidence and tape recordings of inducements and threats made to the original source of the stolen emails and how our emails were later then accessed in full cooperation of other third parties that the police have been informed of.”

Banks is a director of a private security company, Precision Risk and Intelligence, which he founded with a former South African policeman. It is not known if Banks instructed a private investigator - and if he did, whether he instructed that firm - to try to obtain private information about Katz – now director of programmes at Channel 4.

Before his appearance at the select committee for the Department of Culture, Media and Sport on 12 June, Banks told the Observer that he had carried out “oppo research” on the MPs on the committee. He said: “We’ve got all sorts of interesting details. One of them is paid £50,000 a year by the Countryside Alliance just to talk about squirrels.”

During the committee hearing, Ian Lucas asked Banks a question about his insurance company and he replied: “Are you the MP that got drunk in the House of Commons and harassed a woman and got drunk on a karaoke evening?” Lucas replied that he wasn’t. He told the Observer that he wasn’t shocked by the personal attacks though he found them disturbing.

“They go for you. It’s to intimidate you. They are bullies. And they go for you when you get them in a corner. The allegations about their meetings and business dealings with the Russians are extremely serious and they are lashing out.”

The Observer has also obtained evidence, from a subject access request, that Banks’s Leave.EU team put out inflammatory social media messages about “rebel” MPs last autumn despite a warning from an employee that they might be “too menacing post-Jo Cox”. The emails suggest Banks appeared to ignore the message and told him to do it anyway.

On 13 December, after a crucial vote on the EU withdrawal bill, Banks emailed a Leave.EU employee and asked him to get a list of Tory MPs who had voted for the amendment to target them and “hit them locally”. Banks asked him to “also look at each Tory MP rebel and see what we can find about them re expenses scandal and any other interests”. The employee emailed back: “Is saying there must be consequences for their actions too menacing post-Jo Cox?” Banks replied: “No we need to push deselection by local branch target chairman etc”.

Several of the MPs targeted, including Anna Soubry, had previously reported death threats to the police after being targeted by the Daily Mail, Daily Telegraph and others including Leave.EU of being a “traitor”.

A pro-Brexit MP has said parliament needs to “urgently” investigate whether Russia has infiltrated the British electoral system. Bob Seely, an expert on Russian hybrid warfare, said: “I support the government but we must take a longer-term view. We know Russia has made alliances on the hard left and the hard right all across Europe. And it’s very concerning that the leaders of Leave.EU have been so dishonest about these links.”

Banks’s MEP, Molly Scott Cato, said she had written to Sajid Javid, the home secretary, demanding to know what had been done to investigate collusion between the Russian security services and the Brexit campaign, in particular what MI5 knows and what information should be made public.