The defence secretary, John Healey, has urged Nigel Farage to provide transparency about the £5m gift he received from a billionaire businessman, in particular over whether any of the sum could have been linked to Russia-connected profits.

In a letter to the Reform UK leader, Healey also asked him to address the possibility that the war against Iran might boost the revenues of AML Global, an aviation fuel company owned by Christopher Harborne, who gave Farage the £5m in 2024. Farage initially supported the US-Israeli attacks on Iran.

The letter, seen by the Guardian, asked Farage to confirm that none of the sum was “derived from transactions with Russian state-linked energy companies”, and to give assurances that AML Global had complied fully with all sanctions on Russian energy since the full-scale invasion of Ukraine in 2022.

In a statement to the Guardian, AML Global said it had complied fully with all UK and international sanctions, and screened any business partners to ensure the same.

Reform UK and Farage were contacted for comment.

The Guardian revealed last month that shortly before the 2024 general election, Farage was given £5m by Harborne, a British-Thai dual citizen based in Thailand.

Farage did not disclose the money at the time, and it only emerged when the Guardian reported it.

He has argued that because it was an unconditional gift, and received before he announced he would run for parliament, there was no need to declare it once he did become an MP.

However, after a complaint from the Conservatives, Farage faces a formal investigation by the parliamentary standards watchdog, Daniel Greenberg, into whether he should have done.

In the letter, Healey noted that AML Global supplies jet fuel through a network of “main and regional oil companies” covering more than 1,200 locations worldwide, including central Asia, the Gulf and eastern Europe.

Healey asked Farage to confirm that none of the profits which helped finance the £5m gift came from transactions with Russian state-linked energy companies, that AML Global had fully complied with all Russia sanctions, and that “no fuel sourced from Russian-controlled refineries has passed through its supply chain”.

The letter went on: “If you cannot answer either question with confidence, will you commit to a fully independent audit of AML Global’s supply chain and publish the results?”

Citing previous comments by Farage about Russia – for example, that Nato “provoked” Russia’s invasion of Ukraine by expanding eastwards – Healey said this wider situation “places Reform UK under a Russian cloud that only transparency can lift”.

On Iran, the letter asked Farage to say whether he was aware of a potential benefit to Harborne’s company from rising aviation fuel prices when he made supportive comments about the attack on Iran, which led to Iran blockading the strait of Hormuz.

Healey added: “The public is entitled to ask whether your financial interests were impacting on your political positioning and your initial support for throwing the UK armed forces headlong into a war in the Middle East without a plan.”

Healey wrote: “I’m not asking you to return the money. I’m asking you to open the books. If the answers are as clean as you would no doubt claim, that transparency will cost you nothing. If they are not, the public has every right to know.”

He added: “I want to be clear: the purpose of this letter is not to make allegations, but to ask questions that the public interest requires you to answer.”

A spokesperson for AML Global said the company was “committed to operating in full compliance with US, UK, EU and UN sanctions programmes and with any additional restrictions required by our business and banking partners.

“We screen all new counter-parties against a database of US (OFAC), UK (OFSI), EU and UN security council sanctions. Counter-parties include suppliers, customers and banks. We also re-screen existing counter-parties on a regular basis.”