A senior UKIP MEP has announced she is defecting to the Conservatives in an open letter complaining bitterly of the "dictatorial" leadership style of Nigel Farage.

Marta Andreasen, an MEP for the South-East region, maximised the political impact of her decision, breaking the news in a letter to the voters of Eastleigh, where an increasingly tight by-election is being fought.

In her letter, published on Saturday 23 February, Andreasen accused Farage of "surrounding himself with an old boys' club of like-minded sycophants", and making the party "his plaything to mould and shape in any way he sees fit, regardless of the views of others, myself included".

She said Farage "treats any views other than his own with contempt", and criticised his decision not to stand in the by-election, saying that under him, UKIP was "only interested in being a pressure group".

Andreasen, 58, had been increasingly vocal in her criticism of Farage, accusing him two weeks ago of bullying, being "anti-women" and "a Stalinist".

Farage responded by calling Andreasen "irresponsible", saying the Tories "deserve what is coming to them".

The defection is a coup for David Cameron, and could tarnish what had been seen as a strong UKIP campaign in Eastleigh, with pollsters predicting the party could beat Labour into third place.

The Conservatives will hope Andreasen's decision could boost their fortunes after what has been a lacklustre campaign on their part.

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.

In her letter, Andreasen said she was partly swayed by Cameron's decision last month to offer voters an in-out referendum on Europe, saying it showed the prime minister "can listen, adapt and do what is right for the country, not just for personal gain".

Andreasen said Farage had failed to show "real leadership", and instead hailed the Tories as the "true referendum party".

Her remarks provoked an angry response from Farage, who cast doubt on her record, saying she had "left the OECD, the European Commission and UKIP in unpleasant circumstances".

Her move surprised some analysts, since only a month ago she called Cameron's EU speech in Brussels "naive", saying the prime minister "obviously doesn't know Brussels".

"Flexibility sounds great and was probably dreamed up by the Prime Minister whilst sitting in his slippers in Chequers," she said, adding: "I can assure the Prime Minister that there is no such thing as flexibility when it comes to the EU's objective: a deeper federal Europe... His speech therefore was naive."

Andreasen, who is Spanish, was elected an MEP in 2009 and sits on six committees in the European Parliament. She previously worked as an auditor and financial director and in 2010 became an accountant at the European Commission.

Andreasen had been UKIP's only remaining female MEP, following the expulsion of Nikki Sinclaire in 2010.

Her relations with Farage had been under considerable strain since her demand two years ago that he step down over the party's poor performance in council elections.