A row has broken out between a pressure group and UKIP over election spending as part of former leader Nigel Farage's failed bid to become a Kent MP.
Hope not Hate has claimed UKIP failed to declare significant spending, saying the total was nearly twice as much as the party was allowed to spend.
Mr Farage strongly rejected the claims and told the BBC he had no reason to think that anything was wrong.
However, one whistleblower said UKIP payments to him were not declared.
In a separate development, Kent Police and the Electoral Commission are currently investigating Tory Party spending on the South Thanet seat - won by Craig Mackinlay for the Conservatives with Mr Farage coming second.
Hope not Hate said it wanted the Electoral Commission and Kent Police to investigate alleged "flouting of electoral law" by UKIP.
The left-leaning pressure group said South Thanet candidates were allowed to spend £15,087.30 in an election period between 9 April and 7 May, but UKIP spent at least £26,000.
It said its investigation found campaign staff, billboards and mobile ad-vans were omitted from the South Thanet electoral returns.
Mr Farage said: "I didn't do the returns myself, but I was deeply conscious the whole way through that we had to be desperately careful."
He added: "I have no reason to think anything's wrong.
"I must have asked the question ten times 'are we absolutely sure we've got this right?' and I was absolutely assured that we had done this as thoroughly and as professionally as we could."
A UKIP worker who asked not to be identified told the BBC he worked on the campaign canvassing for the party in the six weeks running up to the election.
He said he was paid more than £1,000 which he believes was not declared, and added: "I just think it's very, very bizarre that I know I was paid that sum of money and it's not registered anywhere.
"I know that I was specifically working in South Thanet so it can't be accounted on the national expense. But even so, it is not registered anywhere that I was paid that much."
The BBC checked returns signed off by Mr Farage and his election agent and was unable to find any entry that matched the payment.
The BBC has also spoken to three young activists who confirmed they were paid to work in South Thanet but it is unclear where their salaries have been declared.
UKIP has said it is confident it complied with all electoral regulations, but the party declined a further interview with BBC South East.
Under election spending rules, parliamentary candidates are limited to a maximum of £8,700 pounds plus up to nine pence for every registered voter, which gave a maximum spend in South Thanet of £15,087.
Overspending, or knowingly not declaring costs, can lead to criminal sanctions.