Robert Jenrick has said it is “legitimate” for the media to ask questions about Nigel Farage’s £5m personal donation from a cryptocurrency billionaire, just days after the Reform UK leader told an interviewer it was “none of your business”.

Jenrick, who is Reform’s shadow chancellor, said voters on the doorstep were not asking about the money given to Farage by the Thailand-based British crypto investor Christopher Harborne.

However, he acknowledged it was not unreasonable for the media to have questions about the £5m, in contrast to Farage’s claim earlier this week that it was a private matter.

At the British Chambers of Commerce annual conference, asked by the host, Sophy Ridge, a Sky News presenter, whether Farage was right to say people did not care about it, Jenrick replied: “I’ve knocked on a lot of doors, trust me, in the course of the May local elections, the byelection, and in my own constituency, and I have to say, in all sincerity, not a single person has raised that question with me.

“It doesn’t mean that it’s not a legitimate question for the media to ask, but it is not one that, in my experience, is on the tip of the tongue of people across the country.”

He added: “But let me answer the question head on. If you ask about influence, there is no donor influencing Reform’s agenda. If you are saying Reform should have a policy on crypto, we should do, it is a significant growth opportunity.

“Nigel was given this gift before he was a member of parliament and it is the case that some people in politics face a very severe security threat, and it is right he should be able to protect himself.”

Farage became tetchy on Tuesday when asked about the donation, which is now being investigated by the parliamentary commissioner for standards and was first revealed by the Guardian in April.

He initially said the money was for security purposes before later saying it was a reward for Brexit. This week he claimed it had not been spent at all but he could buy Ferraris or gamble it on horses if he wanted.

On Wednesday, Anna Turley, the chair of the Labour party, said she had asked the Financial Conduct Authority to investigate whether Farage’s public pronouncements about cryptocurrencies may have been intended to benefit Harborne.

Farage’s uncomfortable interviews on the subject coincide with the party slipping slightly in the polls while Labour has gained since Keir Starmer announced his resignation, with Andy Burnham his most probable successor as prime minister.

There are also some tensions within Reform over its strategy against rival rightwing party Restore Britain and unhappiness over its choice of yet another candidate with controversial social media posts in the Makerfield byelection.