Ms Andreasen became a Euro MP earlier this year
The treasurer of the UK Independence Party has resigned following a disagreement with a colleague.
Marta Andreasen, who in 2002 was sacked as European Commission chief accountant after criticising its practices, has run UKIP's finances for two years.
Ms Andreasen, who became a Euro MP this year, fell out with party chairman Paul Nuttall over an internal staffing issue, the BBC's Chris Mason said.
UKIP leader Nigel Farage announced last week that he was standing down.
He said he needed to devote his time to running for the UK Parliament at the next general election, where he hopes to unseat Commons Speaker John Bercow, the MP for Buckingham.
Ms Andreasen was elected as a UKIP MEP for the South East of England in June.
Her resignation will be seen as a distraction for the party as it campaigns for a "No" vote in the forthcoming referendum in Ireland on the European Union's Lisbon Treaty.
But Mr Farage told the BBC he had not expected "Marta to stay on very long as party treasurer anyway, as she is now an MEP and busting EU fraud is a full-time job".
He added: "She is on the budget control committee here in the European Parliament - and that's the last place the European institutions want her to be.
"That job was always going to take priority over her being treasurer. As I have learnt myself, I have found being the leader of the group in the Parliament and party leader in the UK and I have found it too much.
"Similarly Marta has a big job here and in the UK and that's really why she's gone."
UKIP, which wants the UK to withdraw from the EU, came second in June's European elections, taking 13 seats.
Ms Andreasen was unavailable for comment when contacted by the BBC.