Mr Farage has thrown his weight behind Lord Pearson
Outgoing UK Independence Party leader Nigel Farage has been accused of "insulting" party members by saying he has only one "credible" successor.
Mr Farage, who is standing down to fight a seat at the UK election, said party treasurer Lord Pearson was "head and shoulders" above other candidates.
But fellow candidate Gerard Batten said Mr Farage was trying to "unduly influence" the leadership poll.
There are five candidates in the race, with the outcome due next month.
Leadership ballot papers were sent to UKIP members earlier this month and they have until 26 November to vote.
The other hopefuls are MEPs Mike Nattrass and Nikki Sinclaire as well as Alan Wood, a councillor from Wiltshire.
Mr Farage, who is standing against Commons Speaker John Bercow at the election, told the BBC's Daily Politics on Wednesday that he was backing Lord Pearson as the only "serious, credible" candidate to succeed him.
Mr Farage, who is remaining as UKIP's leader in the European Parliament, said any other winner would make UKIP's future situation difficult.
"If Lord Pearson gets that job and I'm leading the party in the European Parliament, then I would argue that UKIP is stronger and will be for several years to come," he said.
"If it's not Lord Pearson, things will be tricky. But I think it will be Lord Pearson. He is head and shoulders above the others."
Mr Batten said he would make a formal complaint to party officials about Mr Farage's comments.
"Nigel's remarks are an attempt to unduly influence the campaign and an insult to the other candidates and our members," he said.
Nigel Farage on party leadership race
"He has created an unnecessary problem for the new leader if that is someone other than Lord Pearson by saying that they they are not a serious, credible leader in advance."
Mr Batten, who was re-elected as an MEP in June, said he wanted to take the party "forward" and focus on other issues beyond withdrawal from the European Union.
"I regard myself as a very serious and credible candidate. A lot of UKIP members share that view."
Lord Pearson, a former insurance broker who was ennobled by Prime Minister Margaret Thatcher in 1990, was one of two Conservative peers to defect to Mr Farage's party in 2007.
The party, which wants the UK to withdraw from the EU, came second in June's European elections, taking 13 seats.
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