Ukip MEP Amjad Bashir, who was suspended by the party and defected to the Conservatives, has heavily criticised his former Ukip colleagues.
Speaking in the Telegraph Mr Bashir said Ukip had become a "party of ruthless self-interest", was "pretty amateur" and had a "ridiculous" lack of policies.
He also said the party was "delusional" about its chances of winning seats in May.
After almost three years as a party member, I realise that Ukip is more concerned with furthering its own interests as a political party than delivering for the British people.
Ukip MEP Amjad Bashir, who was suspended by the party for "extremely serious" issues, has defected to the Conservatives.
Prime Minister David Cameron told the Telegraph he was "absolutely delighted" at the Yorkshire and Humber MEP's decision.
News of the defection emerged shortly after Ukip suspended the MEP, who was the party's communities spokesman, over allegations including "unanswered financial and employment questions".
A senior Conservative source dismissed the move as a "desperate attempt" to play down the impact of Mr Bashir's decision.
Ukip MEP Amjad Bashir has been suspended pending investigations into "extremely serious" issues including unanswered financial and employment questions, a party spokesman said.
The spokesman added: "The UK Independence Party has a zero-tolerance policy and takes the matters at hand extremely seriously. The allegations against Mr Bashir are of a grave nature and we will be forwarding our evidence obtained so far to the police.
UKIP will not tolerate anyone abusing their positions in the party, as we have a firm commitment to differing ourselves from the existing political classes."
A former Ukip councillor on trial accused of electoral fraud has denied forging signatures on nomination forms - including those of his own grandmother, father and stepmother.
Matthew Smith, an ex-parliamentary hopeful for Great Yarmouth, is standing trial at Norwich Crown Court accused of six counts of making a false statement in nomination papers in May 2013.
Seven out of eight forms submitted by UKIP in that campaign were alleged to contain forged signatures and all 10 signatures on the nomination form for Magdalen ward candidate Matt Swann - including the three members of Smith's family - were found to be fake.
Smith, 27, of Gorleston, Norfolk, was asked in court to describe some of the people he is supposed to have collected signatures from.
He replied: "If you're going round an estate of mostly elderly people, one looks very similar to another - I know that sounds cruel but it's true."
In many of the cases, he was not personally responsible for collecting the signatures so could not say why they did not match records like passports and driving licenses.
Smith also denies three charges of making false nominations papers.
Two other men - UKIP member Michael Monk and Daniel Thistlethwaite are also on trial. Each deny one charge of making a false statement in nomination papers.
The trial continues.
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The Home Secretary has said comments by Ukip leader Nigel Farage about the Charlie Hebdo attack in Paris were "irresponsible."
Mr Farage said the attack was the result of "having a fifth column" living in Western countries opposed to their ideals.
He told Channel 4 News:
There is a very strong argument that says that what happened in Paris today is a result - and we've seen it in London too - is a result I'm afraid of now having a fifth column living within these countries.
We've got people living in these countries, holding our passports, who hate us.
Luckily their numbers are very, very small but it does make one question the whole really gross attempt at encouraged division within society that we have had in the past few decades in the name of multiculturalism.
Theresa May said it was "irresponsible to talk about a fifth column."
When asked about Ukip leader's comments, David Cameron said: "Today is not the day to make political remarks or arguments.
"Today is the day to stand four-square behind the French people."
Ukip leader Nigel Farage has said he is giving up alcohol for January.
The politician, often pictured with a pint in his hand, said he "needs a break" from drinking after the festive season.
In an interview with Sky News' Dermot Murnaghan, he said:
I'm not being particularly virtuous about this, I just need a break. And it does us all good to have a break now and then.
But, he added, he will not be giving up his cigarettes just yet.
Ukip general secretary Roger Bird has said he is "very glad" to have been cleared by an inquiry of claims of sexual harassment.
The probe supported Mr Bird's argument that his relationship with party candidate Natasha Bolter was "consensual" and did not compromise the candidate selection process.
However he and the party have agreed that he should stand down as general secretary because of the adverse publicity generated by the situation.
Mr Bird said:
I am very glad that the party has investigated and dismissed the allegations of sexual harassment and any impropriety regarding the selection of Ms Bolter as a candidate.
I wish Ukip every success in the election campaign. I remain a member and keen supporter of the party and I will continue to make every effort to help our candidates to victory in May.
Ukip general secretary Roger Bird has been cleared by an inquiry of wrong-doing over his relations with former candidate Natasha Bolter, but has agreed to leave his post early by mutual consent, the party said.
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