Ukip has claimed a Conservative parliamentary candidate has defected to the party.
But the Tories say Mike Whitehead, who is standing in the Hull West and Hessle constituency, was sacked last week.
Nigel Farage described the news as "another hammer blow to Tory pretensions in the north of England".
But a Conservative Party spokesman dismissed the party's announcement as "cynical, misleading and utterly calculating to try and score political points".
He said: "This man is not our candidate for Hull West and Hessle. He was sacked last week.
"He refused to support the local Conservative council candidate and so we wrote to him last week to say that his position was untenable and he could not stand for us at the general election.
"We were already selecting a new candidate for this constituency.
"This is typical Ukip - cynical, misleading and utterly calculating to try and score political points."
Mr Whitehead is a councillor on East Riding Council.
He said he was "disgusted" with the behaviour of the ruling Tory group in East Yorkshire and the "wilful refusal" of the Conservative party to intervene at a national level.
He insisted the "secretive" council needed to be "opened up" and made more transparent.
Responding to the news, Mr Farage said: "I am delighted to be welcoming Mike to the party at this exciting time.
"His move to Ukip just underlines that today the real party of opposition to Labour in the north is Ukip.
"It is another hammer blow to Tory pretensions in the north of England".
Mr Whitehead said he had been increasingly worried about the behaviour of the controlling group of Tories since 2011, when seven Conservative councillors from Haltemprice and Howden resigned, citing bullying and intimidation.
He added: "In my opinion, the behaviour of the ruling group has only gotten worse since then.
"Now, that same group have managed to gain control of the local party structures by ensuring they are responsible for the organisation of the local election campaign.
"However, the Conservative Party at national level has declined to get involved in what it sees as a remote internal squabble out in the shires.
"The total power of single party control without effective opposition is never a good situation as we have seen from recent events elsewhere in Yorkshire.
"I could not in good conscience continue supporting the local Conservative party when they are obsessed with going down the same road."
As a member of Ukip he would be better able to speak up for residents and represent their views on the council, he added.
He went on: "I also hope that with the election of other Ukip councillors, I can help open this secretive council up and make it more transparent and work better for the residents rather than to the benefit of the few."
He said he wanted to ensure the local authority works more closely with other agencies including Hull City Council and Humberside Police.
"It is important that we remove petty fiefdoms and ensure this region is not left behind when greater resources are devolved by the next Government," he added.
Hull West and Hessle is safe Labour territory - former home secretary Alan Johnson is defending a 5,700 majority from the 2010 general election when the Conservatives finished third.
The Conservative Party has forwarded the Press Association a copy of an email it says was sent to Mr Whitehead on Friday.
It is from Conservative nominating officer Alan Mabbutt and bears Friday's date.
He begins by repeating a response to a previous email from Mr Whitehead in which he confirmed his intention to stand for the party in Hull West and Hessle.
It reads: "As you have stated that you intend to stand in the local election, despite not having been selected for the party, and despite Mrs Clarke asking you to agree to support whoever is properly selected by the association to fight the Willerby and Kirkella ward, I am forced to act to protect the party by refusing to agree to you being nominated to represent the Conservative party at the 2015 general election as candidate for Hull West and Hessle."
Mr Mabbutt then adds: "It [the previous correspondence] was drawing attention to the fact that you were not selected as a Conservative for the local elections so could only be standing against the party's selected candidates."
Labour shadow cabinet minister Jon Trickett said: "This is another huge blow for David Cameron's authority. Even his candidates know he cannot win a majority.
"Ukip and the Tories increasingly share the same people as well as the same policies. Both stand for increased health service privatisation, extreme spending plans which threaten the NHS and further tax breaks for those at the top.
"David Cameron will not rule out working with Ukip, but this is clearly what his party wants. David Cameron must now come clean and tell us what his plans are to do a deal with Nigel Farage and Ukip."
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