The UK Independence Party (UKIP) has been warned it faces legal action in an attempt to force it to pay back £367,697 of "impermissible" donations.
UKIP says it reviews its accounting systems twice a year
The Electoral Commission says the party is also facing a £1,500 fine for filing its accounts late.
The donations concerned are thought to have come from a British businessman who is not on the electoral register.
UKIP said its accounting systems were reviewed twice a year to ensure they complied with legal requirements.
It said: "As in the previous year, the party has arranged an audit of the accounts by independent auditors under Electoral Commission guidelines."
The 2000 Political Parties, Elections and Referendums Act forbids taking money from certain "impermissible donors".
People or organisations not registered in the UK usually cause problems.
Parties have up to 30 days from the date the donation was received to return the money to the donor.
The Electoral Commission said it was launching a review of UKIP's "systems for dealing with its financial affairs and meeting statutory reporting requirements".
It is also preparing to take legal action against the Socialist Labour Party.
The move concerns more than £5,090 of donations.