UKIP is under investigation over claims that it breached laws governing party funds.
The Electoral Commission is looking into whether UKIP accepted "impermissible donations" from the Alliance for Direct Democracy in Europe (ADDE) and the organisation which is affliated with it, the Initiative for Direct Democracy in Europe (IDDE).
An audit discovered ADDE financed polling in the UK during last year's general election in a number of areas - including pre-election surveys in Thanet South, where Nigel Farage stood for Parliament. Nine opinion polls ahead of the 2015 election and 2016 referendum were found to have breached the rules on European party financing.
Expenditure linked to the services of three consultants was also considered non-eligible by an external auditor and by the Parliament's administration.
The European Parliament Bureau has already ruled the ADDE, which UKIP belongs to, will have to repay 172,655 euros (£146,696), and will lose its grant of 248,345 euros (£211,000) after the bureau found they had misused EU funding.
In a statement the Electoral Commission said: "ADDE and its affiliate IDDE, as with other European political parties and foundations, can receive grant funding from the European Union (EU).
"It cannot, however, be used for a range of other specified purposes, including for the direct or indirect funding of national parties, election candidates and political foundations at either the national or European level."
"The European Parliament has formally concluded that ADDE and IDDE used EU grant funding for the benefit of UKIP in breach of its rules and therefore, these expenses were declared as non-eligible for the financing".
"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."