A senior UKIP MEP has resigned as deputy whip in the European Parliament over support within the party for an anti-Islam campaigner.
Anne Marie Waters, who founded the Sharia Watch pressure group and has called Islam "evil", has been allowed to stand in UKIP's leadership election.
Mike Hookem said he was not prepared to "turn a blind eye" to extremism.
He resigned after UKIP chief whip Stuart Agnew declared his support for Ms Waters.
In a letter to interim UKIP leader Steve Crowther - sent before Ms Waters was passed by the party's national executive as a leadership candidate - Mr Hookem said he was "disturbed" by Mr Agnew's support.
In a later statement, he elaborated: "I strongly disagree with the views Ms Waters and Mr Agnew promote and I would like to put as much distance between me and them as possible."
He said he was not a racist and would not support someone "who seeks to single out a section of our society simply due to their religious beliefs".
'Doesn't make sense'
Mr Hookem subsequently told the BBC News Channel: "I believe she should not be in the party and she should not be standing as a candidate.
"She could not stand for a Westminster seat because she was not a member of good standing.
"She could not stand as a Police and Crime Commissioner as she was not a member of good standing - and now suddenly she's standing for the top job."
Mr Hookem continued: "It doesn't make sense. God forbid she does become leader."
He said he would not quit the party if Ms Waters was elected, but he would not support her.
Ms Waters believes her anti-Islam message, including a proposed ban on the burka, the closure of all Sharia councils and a temporary freeze on all immigration, will strike a chord with many voters.
But fellow MEP Nathan Gill has said he would leave the party if she became leader.
"She shouldn't be a member of this party, but we are where we are," he said.
"If she was elected, I would have to leave."
Former leader Nigel Farage has warned that UKIP will be "finished" if it becomes an anti-Islam party.
But the party's deputy leader, Peter Whittle, said it was right that Ms Waters be allowed to stand.
Mr Whittle - who is also competing to be leader - said she had passed the vetting process.
"If you get through that then you should be allowed to stand," he said.
Mr Whittle told the Today programme he was concerned by the number of Sharia courts in the UK.
"There is one law in this country and that is British law," he said, denying it was an anti-Islamic viewpoint.
He added that he was for a "cohesive multi-ethnic society ... united under British values and British laws", adding that multiculturalism had led to "a more fragmented society".
Voting papers will be sent to party members over the next few weeks and the new leader will be announced at the party's annual conference in Torquay, held on 29 and 30 September.
Mr Whittle and Ms Waters are among 11 candidates vying to be leader after Paul Nuttall - who has previously said Ms Waters' views made him "uncomfortable" - stood down after UKIP's poor performance in June's general election.
The 11 candidates are (in alphabetical order):
- David Allen
- Henry Bolton
- David Coburn
- Jane Collins
- David Kurten
- Marion Mason
- Aidan Powlesland
- John Rees-Evans
- Ben Walker
- Anne Marie Waters
- Peter Whittle