A UKIP MEP has defected to the Conservatives, but his former party said it had suspended him over financial and employment concerns.
Amjad Bashir said UKIP had become a "party of ruthless self-interest", was "pretty amateur" and had a "ridiculous" lack of policies.
Party leader Nigel Farage said there were "extremely serious" questions which Mr Bashir had not answered.
Mr Bashir rejected the claims as "absurd and made-up allegations".
The MEP for Yorkshire and the Humber, who was also UKIP's communities spokesman, had been expected to confirm his defection to the Conservative Party on Saturday evening but, shortly beforehand, his suspension from the party was announced.
He had met David Cameron on Friday to discuss the move, and reported the prime minister told him he was "absolutely delighted" that he was rejoining the Tories.
Mr Bashir had previously been a Conservative Party member, and became involved with UKIP three years ago.
But he said the party was "delusional" about its chances of winning seats in May's general election.
Mr Bashir has in the past defended the party against accusations of racism and insisted race and religion had nothing to do with his decision to leave UKIP.
UKIP said Mr Bashir was being investigated for matters including "unanswered financial and employment questions" and "interference with UKIP candidate selection processes".
A UKIP spokesman said the party had 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," he added.
In a statement on its website, UKIP also said one of the reasons Mr Bashir had been suspended was his "continued affiliation" with Mujeeb Bhutto, who was involved in a Pakistani kidnapping gang.
Mr Bashir told the BBC: "I would say these are old historical allegations that the party already knows about. Nigel Farage spoke on TV to defend me against these false allegations.
"These are just dirty tricks to try and discredit me."
BBC political correspondent Iain Watson said: "The Conservatives are standing by their new member - and say that in effect he defected when he met the prime minister. They say UKIP's allegations are an attempt to smear him.
"UKIP says Mr Bashir failed to turn up to a meeting to discuss the allegations with Nigel Farage on Tuesday."
In an interview with the BBC carried out before the news of his suspension, he said controlled immigration and an EU referendum were "not achievable with UKIP".
By Jim Reed, BBC News
When Mark Reckless resigned in September 2014, becoming the second Conservative MP to defect to UKIP, the game of "who's next" got into full swing in Westminster.
No one really expected a move in the other direction.
Amjad Bashir's decision is important for a number of reasons. He is one of the party's more confident media operators - able to hold his own in broadcast interviews.
As the son of Pakistani immigrants he is often held up as evidence UKIP is far more than the "BNP in blazers" that its detractors try to paint them to be.
For David Cameron this is a gift from above four months before the general election.
The PM, who is understood to have met Mr Bashir for an hour at his house in Oxfordshire last week, is said to be delighted at his decision.
At one point David Cameron looked like he was struggling to stem the defections from his own party to UKIP.
Now he's shown that this process does not have to be one-way traffic.
The defection is thought to have been brokered by another Tory MEP, the heavily Eurosceptic Daniel Hannan.
There are already rumours that at least one other UKIP MEP is considering their position following Mr Bashir's resignation.
Mr Bashir told the BBC: "I think it is right that I make good the promises I made to my electorate and join a party that is able to deliver those promises.
"The only party that is able to and is offering the referendum is the Conservative Party and they have the ability to deliver that."
Mr Bashir, a former restaurateur who has also acted as UKIP's small businesses spokesman, revealed that he would not be standing down in order to prompt a by-election.
Defending that decision, he said: "They voted for the policies that UKIP have, and those policies I can best deliver from the Conservative Party."
He described UKIP's campaigning against the Conservatives as "self-centred" since it would, he claimed, "cause a significant dent in the Conservative vote... and let in Labour, and Labour doesn't propose a referendum nor the immigration controls that we want".