West Midlands MEP Mike Nattrass has quit the UK Independence Party in protest at leader Nigel Farage's "totalitarian" leadership.
Mr Nattrass, a former deputy leader of UKIP, has been involved in a legal row with the party hierarchy over his deselection as a candidate at next year's European elections.
He claimed only "cronies" of Mr Farage were being picked to stand and told Channel 4 News: "The process has been totally gerrymandered and fixed."
Mr Nattrass added: "I belong to a party that now is totalitarian and it will not let the voter speak, because not only have they decided who will stand, they have also decided that the party members who should have a free vote will not get a free vote."
Mr Farage is an "excellent frontman" but a "very bad leader", said Mr Nattrass, who could stand as an independent next year.
"He only wants people in the party who absolutely and totally agree with him, so I'm regarded as a troublemaker."
A UKIP spokesman said the party was only made aware of Mr Nattrass's intention to resign because of his television interview.
He said: "I understand his disappointment at not being in the UKIP lists for the forthcoming election, but the shortlist was decided by a fair and comprehensive process and UKIP now has very many talented people coming forward, wishing to represent us.
"Mr Nattrass took his grievances over the selection process to the High Court and lost. His Honour Judge Perle QC ruled that not only was the UKIP selection process fair but it was designed to be fair.
"The process is now handed over to the UKIP membership who get to vote for their preferred candidates to decide the order of the lists which go forward.
"Mike announced publicly two years ago that he thought this would be the right time to retire. We wish him well."
• Another West Midlands MEP, Nikki Sinclaire, was expelled by UKIP in 2010 after resigning from the Europe of Freedom and Democracy alliance, to which UKIP belongs.