The senior Ukip MEP Amjad Bashir says he has defected to the Conservatives shortly after Nigel Farage's party announced that it had suspended him for '"extremely serious" issues relating to finance and employment.
As David Cameron told the Telegraph was "absolutely delighted" by the move, Mr Bashir told the newspaper that Ukip's claims were a "desparate attempt" to play down the defection.
He said: "On Friday I met David Cameron and applied to join the Conservative Party. It is clear Ukip's action today is a desperate attempt to spoil this and is without any foundation.
"The issues raised in my notice of suspension are historic and well known to the party. Indeed, on one of them, Nigel Farage has publicly defended me over it."
Mr Bashir told the Telegraph that Ukip had become a "party of ruthless self-interest", was "pretty amateur" and had a "ridiculous" lack of policies.
He said the party - which has seen its opinion poll rating soar and topped the national poll in last year's European elections - was "delusional" about its chances of winning seats in May.
He said: "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.
"I've seen Ukip both at home and abroad, and I'm sorry to say they're pretty amateur. In the European Parliament, some of their MEPs think it's acceptable to shout and fool around."
He added: "They think they'll sweep up dozens of seats in May, but that's delusional. What they are in very real danger of doing, however, is making a big enough dent in the Conservative vote to let Labour in."
The coup is a major boost for the Prime Minister, who has suffered the defection of two MPs to Ukip and has seen the Eurosceptic party eat into Tory support.
Mr Cameron told The Telegraph during a meeting with Mr Bashir that he understood why voters had been tempted to Ukip.
"I understand why some people have drifted away to Ukip," Mr Cameron said. "I want to genuinely win those people back by saying look, we are the only party that can offer the strong immigration control this country needs; we are the only party that can deliver that referendum that the British people deserve.
"And crucially, this general election is not a by-election, it's not a beauty contest, it is not a chance to send a message or make a statement, it is about choosing a government of the United Kingdom."
Mr Cameron said: "I'm absolutely delighted that Amjad has decided to leave Ukip and join the Conservative Party."
Mr Bashir, who is a Muslim from a Pakistani background, had an "inspiring" story, Mr Cameron said.
He added: "It's another sign that in this great country of ours you can come to Britain without very much and you can be a member of the European Parliament, an MP, sit in the Cabinet."
News of the defection emerged shortly after Ukip issued a statement saying it had suspended Mr Bashir and would be passing information from its investigation to the police.
A party source said Mr Bashir had been aware for some time that he was being investigated and suggested that the suspension was the reason for his defection.
"He knew he would be receiving that letter today," the source said.
Mr Bashir had been informed he was facing investigations that include unanswered financial and employment questions, interference with Ukip candidate selection processes and a continued affiliation with Mujeeb Bhutto, a former Ukip spokesman who resigned from the party after it emerged he had been jailed for his part in a Pakistani kidnapping gang.
A Ukip spokesman said: "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. As a result, Mr Bashir's involvement with the party was suspended today with immediate effect pending further investigations."
But Mr Bashir said: "On Friday I met David Cameron and applied to join the Conservative Party.
"It is clear Ukip's action today is a desperate attempt to spoil this and is without any foundation.
"The issues raised in my notice of suspension are historic and well-known to the party. Indeed, on one of them, Nigel Farage has publicly defended me over it."
Mr Bashir's defection was welcomed by his new colleagues in Brussels.
Ashley Fox, leader of the Tory MEPs, said: "I am a great admirer of Amjad. He is a straightforward, down-to-earth character and he has an energetic and practical approach to politics.
"Having seen Ukip close up, he knows they cannot deliver anything the British people want or need. Amjad sees that only the Conservatives can deliver reform, renegotiation and a referendum in Europe and he will be joining me and my colleagues in campaigning hard to return David Cameron to Number 10 in May."
Syed Kamall, leader of the European Conservatives and Reformists Group in the European Parliament, said: "Like us, Amjad believes in genuine equality of opportunity and the right of people to go just as far as their talent and hard work will take them.
"He is a natural Conservative and I am delighted that he is back where he belongs."
Additional reporting by Press Association
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