Ukip misspent almost half a million pounds of EU funding on trying to win the Brexit vote and elect MPs, a leaked Brussels audit has found.

Nigel Farage’s party broke spending rules by diverting taxpayers’ cash to its own polling ahead of the EU referendum and in key target constituencies for last year’s general election.

EU funding is meant to be spent on a party’s activities at a European level, not on financing domestic campaigns.

If the conclusions of the external audit are confirmed next week, Ukip could be forced to repay a huge sum – while being barred from claiming further Brussels money.

And that will come at the worst possible time for the cash-strapped party, which has moved out of its London headquarters and whose key donor, Arron Banks, has threatened to end his funding.

The leaked audit – obtained by Sky News – focused on money provided to the Alliance for Direct Democracy in Europe, a European political grouping dominated by Ukip.

It found ADDE financed polling in the UK between February and December last year, judged as “indirect financing of a national political party” and “a referendum campaign” – both of which are prohibited by European Parliament rules.

According to Sky, the report concluded: “The constituencies selected for many of the polls underline that the polling were conducted in the interest of Ukip.

“Most of the constituencies can be identified as being essential for reaching a significant representation in the House of Commons from the 2015 General Election or for a positive result for the ‘Leave campaign’.”

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And on the EU referendum the auditors said: “Several polling can be considered as financing of a referendum campaign which violates 8(4) ... prohibiting the financing of referenda campaigns.”

The audit, drawn up for the European Parliament Bureau, put the total misspend at over €500,615.55 (£430,486.82) by ADDE, which includes other parties in Europe.

However, an EU spokesperson said the “lion's share” was by Ukip, amounting to over €450,000 (£386,961).

If the bureau, next Monday, agrees with the conclusion of the external audit, Ukip could be forced to pay back more than €170,000 (£146,185), while not being able to claim hundreds of thousands more.

The money, according to the audit, was used to fund polling in Great Grimsby and Thurrock, Rochester and Strood and Cardiff South and Penarth, all Ukip target seats at the last election.

Polling was also paid for ahead of the General Election in Thanet South, where Nigel Farage unsuccessfully ran to become an MP.

And it continued after the election, to fund several EU referendum attitude polls across the UK.