A Met spokesperson said it had received "a number of reports relating to donations and polling involving a political party", after undercover footage was broadcast by Channel 4 News and Verbatim Investigations.
The police investigation by the Met's Special Enquiry Team is being folded into an inquiry that has been under way into other donations to the party.
A Reform UK spokesman said the party "denies any wrongdoing and will fully cooperate with the investigation".
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