UKIP MEP Amjad Bashir has defected to the Conservative Party, with David Cameron saying he was “absolutely delighted” with the decision.
But Nigel Farage’s party said it had suspended him over allegations of a “grave nature”, and would be forwarding its evidence to the police.
A senior Tory source, however, dismissed the move as a “desperate attempt” to divert attention from his decision to quit UKIP.
Mr Bashir told the Telegraph that UKIP had become a “party of ruthless self-interest” and was “pretty amateur”.
He added that it was “delusional” about its chances of winning Parliamentary seats in the general election in May.
Mr Bashir told the newspaper: “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 a statement, he added: “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 it 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.”
News of his defection emerged after UKIP announced it was suspending the MEP, who represents Yorkshire and the Humber, pending investigations into “extremely serious” issues.
Mr Bashir had been informed of the allegations, which include claims of interfering in the candidate selection process as well as unanswered financial and employment questions, the party claimed.
The 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.”
Mr Bashir, UKIP’s communities spokesman, reportedly held a “continued affiliation” with Mujeeb Bhutto – who resigned from the party in 2014 – despite official protests.
The defection is a boost for David Cameron, who saw two of his own MPs defect to UKIP last year.
He said: “I’m absolutely delighted that Amjad has decided to leave UKIP and join the Conservative Party.”
A UKIP spokesperson said Mr Bashir promised in his resignation letter that his decision was “not intended to cause any embarrassment for UKIP”, and that his defection was “a principled decision”.