Nigel Farage is under mounting pressure to provide evidence for his claim that a state-sponsored Russian hack was behind the disclosure of the £5m gift he received from the crypto billionaire Christopher Harborne.

Reform UK claimed over the weekend that analysis of Farage’s phone by “counter-espionage experts” suggested that “Farage’s phone, email and bank accounts were compromised by hostile actors, almost certainly linked to Moscow, using spear phishing tactics”, before the Guardian revealed details of his undeclared gift last month.

Farage told the Mail on Sunday that the alleged Russian activity was “deeply concerning” and highlighted the “threat they pose to British security”.

Farage’s claims followed intensifying scrutiny of the £5m gift he received from Harborne. Farage had not publicly declared the gift, despite parliamentary transparency rules requiring MPs to register gifts and donations above a certain threshold.

A spokesperson for the Guardian described Farage’s claim as “an attempt to deflect attention from legitimate scrutiny of his financial affairs”. They added: “Nigel Farage is once again hiding behind a baseless attack on the media rather than facing up to scrutiny from journalists and politicians.”

The spokesperson said it was absurd for Farage to suggest it had found out about the gift from a Russian hack.

After Farage made the allegation on Sunday, Labour and the Conservatives called on him to hand any evidence he has to Britain’s security services.

Farage’s spokesperson did not respond to questions from the Guardian over who he had reported the alleged hack to, whether any evidence was handed to British authorities or how the examination of his handset had suggested that Russia was responsible.

It is understood the National Cyber Security Centre is not aware of a report from Farage related to the alleged hack.

The Guardian has contacted Reform UK for comment.

Kevin Hollinrake, chairman of the Conservative party and MP for Thirsk and Malton, told the Guardian: “This is what you get when you sup with ‘The Devil’. Nigel Farage spent years making excuses for [Vladimir] Putin, now, conveniently, he is suddenly playing the Russia card, just as serious questions are being asked about his failure to declare a £5m donation from a crypto billionaire based in Thailand.

“We should not forget that this is the same party that has faced persistent questions about its stance on Russia, including its former leader in Wales who was jailed for taking bribes from the Kremlin.

“If he has genuine evidence that Russia attempted to hack him, he should immediately hand it to the relevant authorities and be fully transparent with the British public about exactly what he knows. The British people are entitled to answers, not distraction tactics.”

A Labour party spokesperson said: “Russian interference in our politics is incredibly serious and all political parties have a responsibility to challenge it head on and ensure any potential instances of foreign interference are investigated.

“Given the seriousness of these claims, Nigel Farage needs to reassure the public that he’s reported this to the security services. He also needs to finally come clean as to how his secret £5 million ‘gift’ from his crypto billionaire backer was spent and why he failed to declare it.”

A Reform source quoted in the Mail on Sunday referred to the £5m as a donation – in contradiction to Farage’s repeated claim it was a ‘gift’ that he did not need to flag to the parliamentary authorities.

The money was given to Farage before he announced in June 2024 that he was running in the general election race. It came through a company linked to Harborne, one of the UK’s wealthiest crypto investors and a major financial backer of right wing causes.

Initially Farage claimed the £5m was given to him for security purposes to keep him “safe and secure” for the rest of his life. He also claimed the gift was “purely private” and “wasn’t political in any sense at all”.

Harborne said he gave Farage the money to support his “security not just now but for the rest of his life”.

But weeks after the Guardian’s inital investigation, Farage told the Sun in an interview that it was “given to me on an unconditional basis, completely unconditional basis. But frankly it was given as a reward for campaigning for Brexit for 27 years”.

The Reform UK leader said the money made no impact on his decision to return to public life.

After a referral from the Conservatives, parliament’s commissioner for standards Daniel Greenberg launched an investigation under rule five of the MPs’ code of conduct, which specifies new MPs should register relevant financial interests received in the 12 months before their election within one month of entering parliament.

Ministers raised concerns over whether some of Harborne’s wealth may have originated from Russian-linked business interests.

John Healey, the defence secretary, last week challenged Farage to clarify whether any of the £5m gift “may have come from Russia-linked profits”.

A Reform source suggested that the explanation for the claimed hack could be that Putin is angry with Farage over his support for Nato. In January, Labour accused Farage of “parroting Kremlin lines” when he said he would vote against any deployment of UK troops in Ukraine after a peace deal.