Nigel Farage’s claim that a Russian hack was behind a Guardian report on the £5m gift he received from a crypto billionaire has been described as “without any merit” by a former head of the National Cyber Security Centre.

Ciaran Martin, founding chief executive of the agency, which is part of GCHQ, said Farage’s allegation, if true, would have major implications for UK policy towards Russia but that the Reform UK leader had yet to provide “a shred of evidence”.

It is understood that Farage is yet to ask the NCSC to investigate his apparent belief that the Guardian’s revelation of the multimillion-pound donation by crypto billionaire Christopher Harborne was the result of a Russian “hack-and-leak” operation.

Martin said such an operation by the Kremlin would amount to an “unprecedentedly aggressive intervention” into Britain’s democracy and that Farage should both contact the NCSC to investigate and make public the technical evidence he has for his claim.

Martin said: “An aspiring prime minister has essentially claimed that Russia has launched an unprecedentedly aggressive intervention – a malicious intervention – in British politics, and he’s not produced a shred of evidence to support that claim.

“He’s made a serious foreign policy and national security allegation which if true would have massive implications for British policy towards Russia.

“It is a very, very serious thing to allege. It would be a national security issue. If it is true, the government should be in emergency session in COBR right now considering their response to the most serious Russian intervention in internal British affairs for years.”

Farage’s allegation followed the launch of an inquiry by parliament’s commissioner for standards into his failure to declare the gift made by Harborne in 2024.

Parliamentary transparency rules require MPs to register political gifts and donations above a certain threshold.

The Reform leader claims Harborne’s gift was exempt as it was both personal and for his security.

Over the weekend, a Reform UK source claimed analysis of Farage’s phone by “counter-espionage experts” suggested “Farage’s phone, email and bank accounts were compromised by hostile actors, almost certainly linked to Moscow, using spear phishing tactics”.

Reform UK told the Mail on Sunday that only four people knew of the gift, with Farage adding that the alleged Russian activity was “deeply concerning” and highlighted the “threat they pose to British security”.

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.”

Martin said there was a cross-party consensus that cyber-attacks on politicians were an attack on the “integrity of our political system, they are attacks on the country and on our national security”.

He said the onus was on Farage to report the matter to the appropriate authorities given the serious implications for the country and its democracy.

He said: “It would require a national government response of some kind, whether it’s further sanctions or the expulsion of diplomats, and so forth. But, as it stands, this is an entirely unsubstantiated claim.

“Who knows what may emerge in the future – of course I cannot say for certain that this didn’t happen and I will change my view if the facts change.

“But that’s true of most claims and the onus is on Mr Farage to provide at least some evidence that we should take this extremely disturbing claim seriously.

“But at this time, based on what he said to the Mail on Sunday, this is an entirely unsubstantiated claim and one without any merit.

“What he has described would not just be hacking by Russia, but what we call a hack-and-leak operation, which is a different thing.

“It is way more serious because it violates international norms, and it’s a direct intervention aimed at destabilising our democracy and our politics in a way that spying isn’t.”

He added: “An aspiring prime minister should treat it with the utmost seriousness and cooperate fully with the National Cyber Security Centre and any other relevant authority; otherwise you should not make these accusations, because they’re way too serious to just be bandied around.”

Martin questioned Reform UK’s claim that a “team of counter-espionage experts” hired by the party had concluded the hack was almost certainly ordered by Russia.

He said: “Let’s deconstruct that a bit. He hasn’t apparently gone to the authorities. Are the ‘counter-espionage’ experts a cybersecurity company with digital forensics capabilities – who are they? What work have they done? Do they exist? He should at least publish some technical evidence.

“To take one phone and say my phone, email and bank account has been hacked, and it’s got the imprint of the Russian state, that’s a hell of a technical leap to do that on the basis of a single device analysis.

“It is not impossible – but in most cases you’d have to do a lot more by way of a substantive provable investigation.”

Martin added that “Russians don’t leave a little flag on a device”.

“You need detailed technical evidence, some of which is sometimes not even available to the private sector,” he said. “That is why for an accusation of this magnitude the only right port of call is the British security services, and in this case, specifically the National Cyber Security Centre in GCHQ.”

Reform has been approached for comment.