The BBC has been criticised for booking Arron Banks, the pro-Brexit billionaire who is the subject of a criminal investigation, to appear on the Andrew Marr show on Sunday.

Banks is being investigated by the National Crime Agency (NCA) after his case was referred to it by the Electoral Commission, which said there were reasonable grounds to suspect Banks was “not the true source” of £8m given to the Leave.EU campaign.

The BBC’s decision provoked widespread condemnation from politicians, lawyers and activists.

Andrew Adonis, a leading remain campaigner, said in a letter to the BBC that Banks’s expected appearance was the result of “a very serious editorial misjudgment, influenced by a culture of accommodation to extreme Brexiteers now deeply embedded within the BBC”.

Caroline Lucas MP and Molly Scott Cato MEP, both Green party politicians, wrote an open letter criticising the decision to allow Banks to “spread misinformation at our expense”.

Here’s the email to the BBC sent by myself and @CarolineLucas explaining why they should reverse their decision to give a platform to Arron Banks



Please continue with your own complaints about this appalling decision@TheGreenParty pic.twitter.com/5vnXwkY0iy — Molly Scott Cato MEP (@MollyMEP) November 3, 2018

Jo Maugham QC, the director of the Good Law Project, tweeted: “How robust Andrew Marr’s questioning is – and he is a good interviewer – is completely beside the point. The interview is an invitation to viewers to choose between what the independent regulator has said and what Arron Banks says. That is a false and dangerous equivalence.”

The BBC spoke to Banks at Gatwick airport on Saturday morning as he returned to the UK from Bermuda.

When asked about the origin of the funds, Banks said: “I certainly won’t be showing you. You know, we’re going to cooperate with the NCA and they’ll have visibility into our accounts.” When pressed further, he said: “Goodbye, I’m not talking to you.”

According to Andy Wigmore, a close associate of Banks, access to the accounts in question has been released to the BBC prior to Sunday’s show, and Banks is expected to refer to a legal opinion to demonstrate that the financial dealings in question were legitimate.

The Electoral Commission said it suspected Banks had tried to knowingly conceal the origin of the money, and that the money was provided through a company based in the Isle of Man.

It said in a report: “Leave.EU, Elizabeth Bilney (the responsible person for Leave.EU), BFTC, Mr Banks, and possibly others, concealed the true details of these financial transactions, including from us, and also did so by knowingly making statutory returns/reports which were incomplete and inaccurate, or false.”

Both Bilney and Banks deny any wrongdoing, with the latter posting a flurry of tweets on Friday evening.

“The Electoral Commission have made a public statement without producing any evidence,” he said. “I am happy to be robustly interviewed.”

In a statement, the BBC said: “There is a strong public interest in an interview with Arron Banks about allegations of funding irregularities in relation to Leave.EU and the 2016 EU referendum.

“The Electoral Commission has laid out concerns about this in public and it is legitimate and editorially justified for Andrew Marr to question Mr Banks robustly about them, which he will do on Sunday morning.”

The Labour MP Ben Bradshaw wrote to Theresa May on Friday to ask whether the government had blocked requests to conduct an investigation into Banks, amid repeated allegations that an inquiry was prevented during her time as home secretary.