A senior member of the UK Independence Party has said she is leaving the party to join the Conservatives.
Marta Andreasen, a South East region MEP, sent her resignation letter to UKIP leader Nigel Farage on Friday.
It comes two weeks after she accused the UKIP leader of bullying and being "anti-women" and "a Stalinist" and threatened to leave the party.
In response, Mr Farage said the Tories "deserve what is coming to them" and added: "The woman is impossible."
Ms Andreasen, 58, said resigning from UKIP to join the Conservatives was not a decision that she took lightly.
In an open letter expressing her dissatisfaction with UKIP, she accused Mr Farage of treating any views other than his own with contempt.
Ms Andreasen said she did not believe the party could offer "real leadership".
She said Mr Farage surrounded himself with "an old boys club of like-minded sycophants", whereas Conservative leader David Cameron had shown "he can listen, adapt and do what is right for the country, not just for personal gain".
The MEP said the prime minster's speech on Britain's relationship with the EU last month had convinced her the Conservatives were the "true referendum party".
Conservative Party Chairman Grant Shapps said he was "delighted" to welcome her to his party.
"She brings a wealth of experience - and a dedication to fight for what's best for the British people in Europe," he said.
Meanwhile, in a statement issued on Friday, the UKIP leader said: "Having left the OECD, the European Commission and UKIP in unpleasant circumstances, the Conservative Party deserve what is coming to them.
"The woman is impossible."
Relations between Ms Andreasen and Mr Farage had been strained for some time, with Ms Andreasen calling for Mr Farage to quit after the party's poor performance in council elections in 2011.
Since then, UKIP's fortunes have improved and it recorded its best performance in a parliamentary by-election in Rotherham in December.
Ms Andreasen, who is Spanish but was born in Argentina, worked as an auditor and financial director for various companies before working as an accountant for the European Commission in 2002.
She later joined UKIP and was elected in 2009. She currently sits on six committees in the European Parliament.
Ms Andreasen had been UKIP's sole remaining female MEP, after Nikki Sinclaire was expelled from the party in 2010.
Ms Sinclaire later brought a sexual discrimination case against the party which was subsequently dropped.
At the time, UKIP issued a statement affirming its "opposition to discrimination on all grounds as prescribed by law and otherwise" and stressed that sexuality was "no bar to involvement or advancement" within the party.