Less than a week before the Eastleigh by-election, Marta Andreasen quit and launched an excoriating attack on Nigel Farage's leadership.
She dismissed the party's chances of achieving real power, and hailed the Prime Minister's speech pledging an in-out referendum on EU membership as a "game changer".
The parting shot, in an open letter from the South East region MEP, comes amid speculation Ukip could perform well in the crucial electoral test.
Mr Farage opted not to stand as a candidate himself, but said in a statement: "(Marta Andreasen) 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."
Earlier this week, in an interview with The Herald, Mr Farage said the campaign for Scottish independence was "dead in the water" and claimed Deputy first Minister Nicola Sturgeon was "out of her depth".
Ms Andreasen said she did not believe Ukip could offer "real leadership".
"Ukip has good people in it at grass roots that care about the future of the United Kingdom and our place in a re-negotiated relationship with Europe," she wrote.
"Unfortunately, their party leader treats any views other than his own with contempt.
"Ukip is his plaything to mould and shape in any way he sees fit, regardless of the views of others, myself included.
"His actions, surrounding himself with an old boys club of like-minded sycophants, are dictatorial in sharp contrast to those of David Cameron, who has shown he can listen, adapt and do what is right for the country, not just for personal gain."
Ms Andreasen said Mr Cameron's keynote address on Britain's relationship with the EU last month had convinced her the Conservatives were the "true referendum party".
"The Prime Minister's speech was a game changer; he promised to turn around the European freight train, which is currently heading towards a deeper federal Europe with no options for Britain," she went on.
"I was also reassured by the financial deal that David Cameron got for Britain on the EU Budget. This is a matter very close to my heart and for which I have fought hard, and it is an important step for Britain in this battle."
By contrast, the MEP said Mr Farage's decision not to stand as a candidate in Eastleigh after former Cabinet minister Chris Huhne resigned in disgrace showed Ukip was only a "pressure group".
She warned Eastleigh voters that Ukip had no chance of victory, and supporting the party risked letting "the Lib Dems in via the back door".
Conservative Party chairman Grant Shapps said: "I'm delighted to welcome Marta to the Conservative Party.
"She brings a wealth of experience - and a dedication to fight for what's best for the British people in Europe.
"Her decisive words on the Prime Minister's EU budget negotiations prove that the Conservatives are the only party who can protect Britain's interests in Europe."