The party has been carrying out "one final round" of vetting to determine who is eligible to stand in the race.
UKIP donor Arron Banks suggested Steven Woolfe will not be on the ballot.
Mr Woolfe's candidacy is in officials' hands after he missed the deadline and revealed he did not disclose a drink-driving ban when he stood to be a police commissioner in 2012.
Also in the race are MEPs Jonathan Arnott and Bill Etheridge, while Huntingdonshire councillor Lisa Duffy and activists Phillip Broughton and Elizabeth Jones have also put their names forward.
There is continued uncertainty about whether prominent MEP Diane James has applied.
The contest has been sparked by Mr Farage's decision to stand down following the UK's vote to leave the EU, saying his "political ambition has been achieved".
The vetting panel had been expected to announce on Tuesday which candidates were eligible to stand under the party's rules after nominations closed on Sunday.
But after the panel met it issued a statement saying the final list of leadership candidates would be revealed on Wednesday at midday.
"An NEC-led panel has today met to determine the eligibility of those candidates that submitted their nomination papers to stand for party leader.
"That panel has now met and with one final round of checking to go the party aims to announce the full list of candidates at midday tomorrow, Wednesday 3 August," a party spokesman said.
To qualify, candidates are expected to have been party members for two years and to have been nominated by 50 members of the party.
UKIP leadership: Runners and riders
Steve Crowther, who left his job as UKIP chairman earlier this week, would not be drawn on whether Mr Woolfe - seen as the frontrunner in the race - should be excluded from the contest for submitting his papers 15 minutes late - a delay that the MEP has blamed on computer problems.
But asked about the issue of the drink-driving conviction - which Mr Woolfe incurred in 2002 but which he did not reveal when he stood to become a police and crime commissioner in 2012 in a possible breach of electoral law - he told the BBC that this was "arguably more serious".
Mr Woolfe got a Â£350 fine and nine-month ban for being drunk in charge of a scooter but insisted his life had moved on by 2012 and the conviction was spent by then.
Amid reports of a power struggle within the party, UKIP NEC member Victoria Ayling tweeted on Tuesday evening to urge her fellow panel members not to "block" Mr Woolfe.
UKIP donor Arron Banks later tweeted: "Tonight's decision to exclude Woolfe is the final straw" and he claimed it represented a "coup" by UKIP MP Douglas Carswell and Neil Hamilton, UKIP's leader in the Welsh Assembly.
Mr Hamilton - who is backing MEP Mr Arnott for the role - has said Mr Woolfe should not be able to run for the party's leadership if his nomination was submitted late.
Meanwhile, Mr Carswell, the party's only MP, has said he will not be endorsing any of the candidates but suggested they needed to consider what UKIP's purpose was after the EU referendum, how it should go about increasing its elected representation and how the party's leadership style needed to change.
The final list of candidates in the running to succeed Nigel Farage as UKIP leader will be announced later.