The UK Independence Party faces legal action to force it to pay back hundreds of thousands of pounds of "impermissible donations".
The Electoral Commission announced it would initiate proceedings to require the hardline anti-Europe party to forfeit £367,697.
The party also faces fines totalling £1,500 for filing its accounts "unacceptably" late, the elections watchdog disclosed.
In a separate move, the Electoral Commission is also preparing to take legal action against the Socialist Labour Party over £5,090 of donations.
Action will be taken under the 2000 Political Parties, Elections and Referendums Act, which forbids taking money from certain "impermissible donors".
That usually means people or organisations which are not registered in the UK.
Under the 2000 Act, parties have up to 30 days from the date the donation was received to return the money to the donor.
Electoral Commission sources said the Ukip donations in question dated from 2004.
The watchdog said it was also launching a review of Ukip's "systems for dealing with its financial affairs and meeting statutory reporting requirements".
The national party is to be fined £1,000 for filing its 2005 annual statement of accounts almost six months after an extended deadline. Its South East branch will be charged a further £500 for presenting its accounts to the Electoral Commission more than six months late.
Read More
MORE ABOUT