Ukip is being investigated by Britain's elections watchdog over claims it breached party funding laws.
The European Parliament Bureau has already ruled the grouping Ukip belongs to, the Alliance for Direct Democracy in Europe (ADDE), will have to repay 172,655 euro (£146,696), and will no longer be given a grant of 248,345 euro (£211,000) previously allocated to it after finding it misused EU funding.
The Electoral Commission has now opened its own investigation into whether Ukip accepted "impermissible donations" from the ADDE and its affiliated foundation, the Initiative for Direct Democracy in Europe (IDDE).
An audit found that ADDE financed polling in the UK between February and December last year, including pre-election surveys in Thanet South, where Nigel Farage unsuccessfully stood for Parliament, and Ukip target seats Great Grimsby, Thurrock, Rochester & Strood and Cardiff South & Penarth.
Nine opinion polls ahead of the 2015 election and 2016 EU 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.
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).
"This funding can cover up to 85% of the parties' eligible expenditure and be used for a range of activity, from administrative functions through to the campaign costs connected to European elections.
"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 had advised the commission that "it 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 statement said: "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."
Commission officials have already met with representatives of the European Parliament and inspected material gathered as part of the audit.