Reform denied any wrongdoing and said it had been the victim of a hoax perpetrated by climate activists.
The governing Labour Party wrote to the police asking them to investigate possible criminal offences, including an alleged proposal to channel a larger overseas donation through a UK intermediary.
The investigation, broadcast by Channel 4 just as Reform's annual conference opened to the public, said two senior figures in Reform had arranged for a US company to pay more than STG30,000 ($A56,378) for three opinion polls commissioned by the party.
Such an arrangement could breach British laws on political funding, which ban foreign donations.
Posing as wealthy donors from the United States with British connections, the undercover team from an investigative group called Verbatim said it spent a year speaking to top officials in Reform about possible donations, culminating in a lunch with Farage.
Channel 4 said Verbatim was an independent organisation founded by journalists from the Centre for Climate Reporting, but that it had verified their findings, which included covertly filmed footage of meetings.
Farage, a veteran Brexit campaigner who is under parliamentary investigation over earlier donations, told LBC radio: "The party has broken no laws. The party has not taken any dodgy money or anything like that whatsoever."
Reform, which has seen its commanding lead in opinion polls falter over undeclared donor allegations, announced an internal investigation and said the two officials featured in the undercover filming - head of policy James Orr and Farage's right-hand man Dan Jukes - had stepped down pending its results.
Orr, a philosophy professor at the University of Cambridge, said in a video on X that Reform had not been involved in commissioning the poll.
"There's absolutely no way this could amount to a benefit or donation in kind at all," he said.
"It wasn't internal party polling. It wasn't Reform UK's poll. It was a public poll arranged by an independent person paying an independent pollster, and those polls were published immediately for the general public."
Farage, 62, is being investigated by a parliamentary watchdog over a STG5 million donation from crypto-billionaire Christopher Harborne in 2024. He denies any wrongdoing but has seen support for his party soften in recent weeks.
The Metropolitan Police said it was aware of the allegations in the broadcast and would assess any information provided to them. This did not amount to the formal launch of an investigation.
The Electoral Commission, which oversees and regulates political finance in Britain, said: "We are considering all relevant information, in line with our regulatory remit, and we are in touch with the Met Police.
"The Electoral Commission has not approved the donor vetting process for Reform, or any other party. Parties are responsible for ensuring their internal processes and controls are fit for purpose."
with AP
