The UK elections watchdog is considering whether to investigate an undisclosed £5m donation received by Nigel Farage before he announced his candidacy at the last general election.

The move comes after the Guardian revealed this week that the Reform UK leader was given the money by the crypto billionaire Christopher Harborne.

In a written response to the Conservative party, the Electoral Commission said it was “aware of this matter and are considering it under our regulatory remit. We will consider all the available relevant information and recommend what, if any, next steps the commission will take.”

Farage had previously stated he did not intend to stand as an MP but reversed his position in June 2024, within weeks of receiving the personal gift from the Thailand-based businessman.

In July 2024, Farage became an MP for the first time. He has since stated he now expects to become prime minister at the next general election.

The Conservatives wrote to the watchdog on Wednesday arguing that Farage should “have declared the donation to the Electoral Commission at the time as a regulated donee”.

Reform UK said the money was given as an “unconditional gift” to Farage, who was then the party’s honorary president and had not publicly committed to standing for parliament. He had used the funds for security arrangements.

Parliamentary rules state that any benefits should be declared within 12 months before taking office, depending on whether they are for political or personal purposes. The rules state: “If there is any doubt, the benefit should be registered.”

The commission said it would respond to the Conservatives by 12 May, after elections in Scotland, Wales and parts of England next week.

The Conservatives have also written to the parliamentary commissioner for standards, Daniel Greenberg, alleging that Farage breached the Commons code of conduct by “failing to register a political donation”.

The Tory party chair, Kevin Hollinrake, said Farage had been “obliged” to declare the gift. “The Conservatives are today referring Nigel Farage to the parliamentary standards commissioner. This £5m from the crypto billionaire Christopher Harborne raises serious questions,” he said.

“What is Nigel Farage hiding? And why does Reform think the rules don’t apply to them? This stinks and Reform should come clean now.”