A Met spokesperson said it had received "a number of reports relating to donations and polling involving a political party", following the broadcast of a film by Channel 4 News and Verbatim Investigations.
"All allegations under investigation relate to potential offences under the Political Parties, Elections and Referendums Act 2000," the spokesperson said.
The statement added: "Detectives continue to liaise closely with the Electoral Commission and Crown Prosecution Service as enquiries progress."
The police spokesperson said detectives were seeking to obtain footage relating to the broadcast.
A Reform UK spokesman said the party "denies any wrongdoing and will fully cooperate with the investigation".
"We won't be commenting further while a live investigation is under way."
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