Ukip is facing an investigation from Britain's electoral watchdog over whether the party accepted an "impermissible donation" from the Alliance for Direct Democracy in Europe (ADDE).
The probe comes after claims the Ukip-dominated ADDE "misspent" £430,000 ($535,000) of EU grants and faces a fine of almost £150,000 from the European Parliament Bureau.
"They have broadened the definition of 'expenditure supporting a political party' so widely as to deny us the right to undertake any activity that might be remotely interesting to ADDE members," an ADDE spokesman said.
"It has become increasingly apparent since Brexit that anything short of 'group think' is no longer tolerated within the EU. Any deviation will see the rules changed and goal posts moved."
The UK's Electoral Commission said it was contacted by the European Parliament on the evening of 21 November.
"The Commission has now opened its own investigation into Ukip to look at whether there has been any breach of UK election law. This includes whether any impermissible donations have been accepted by the party," a spokesperson for the watchdog said.
"Donations and loans to political parties are regulated under the Political Parties, Elections and Referendums Act 2000 (PPERA). A donation is money, goods or services given to a political party without charge or on non-commercial terms, with a value of over £500."
The watchdog said the time taken to complete an investigation varies on a case-by-case basis.
"Once the investigation is complete, the Commission will decide whether any breaches have occurred and if so what further action may be appropriate," a spokesperson said.
Ukip had not responded to a request for comment at the time of publication.