Diane James, a UKIP MEP, is the new favourite after the campaign of the frontrunner Steven Woolfe ran into difficulties.
James, the party's justice and home affairs spokesperson, today (Wednesday) confirmed she was in the running for the post, having shot to the top of the bookmakers' odds on Tuesday.
Woolfe's leadership bid was rejected after he missed the application deadline by 17 minutes. He had also failed to disclose a spent conviction for drink driving when running to be a police and crime commissioner in Manchester.
RELATED CONTENT
Of the other candidates, James is said to have the most frontline political experience and came close to winning the Hampshire seat of Eastleigh for Ukip in a UK by-election against the Lib Dems in 2013.
The leadership contest was triggered after Farage decided to stand down after the EU referendum.
Although Ukip failed to win more than one seat at the last UK general election, it secured around four million votes and came top in the European parliament elections the year before.
James tweeted on Monday: "Some talk of making @UKIP more 'professional'. I want the next leader to make it so we can win seats locally and in Westminster #WinningisKey."
She has a reputation as a competent speaker and media performer but is also a controversial figure, previously praising Russian President Vladimir Putin for his 'nationalist' leadership.
She said, "I admire him from the point of view that he's standing up for his country. He's very nationalist. I do admire him. He is a very strong leader."
During the Eastleigh by-election, she claimed new migration from Romania was a problem because of the "crime associated with Romanians". She later apologised, saying she had not meant her comments to be a "generalisation against every single individual born with a Romanian passport."
The other candidates include Lisa Duffy, a key party organiser, MEPs Jonathan Arnott and Bill Etheridge, as well as Phil Broughton, a former parliamentary candidate, and Liz Jones, deputy Chair of Ukip Lambeth.
Etheridge is another controversial candidate after he was caught on film suggesting that young activists copy Hitler's "forceful" manner of public speaking. He also wrote a book celebrating golliwogs after he was suspended by the Tories in 2011 over a Facebook picture holding one of the toys to make a point about political correctness.
Arnott, also an MEP, is a former general secretary of Ukip.