She became the party's first female leader last month
UKIP leader Diane James has quit her job just a few weeks after taking over from Nigel Farage.
She became the party's first female leader on 16 September after a landslide victory in the leadership contest.
In a statement, she said it was with "great regret" and that the decision was down to "personal and professional reasons".
Ms James said she would not be "formalising my recent nomination to become leader of the party with the Electoral Commission".
She added: "I have been in discussion with party officers about the role.
"It has become clear that I do not have sufficient authority, nor the full support of all my MEP colleagues and party officers to implement changes I believe necessary and upon which I based my campaign."
The MEP for South East England was catapulted to frontrunner in the UKIP race after deputy leader Paul Nuttall refused to join the race and second favourite Steven Woolfe was disqualified.
She became an unlikely favourite, winning the support of Mr Farage and the party's main donor, Arron Banks, who helped fund her campaign.
In her victory speech she said the party had to make sure they were "race ready" for the next election and appealed for members to come together to deliver a credible manifesto.
She went on to accuse the British prime minister, who she called "magpie May", of stealing UKIP policies and said the Conservatives should remember where the "best ideas came from".
Nigel Farage could face calls from members to return as leader, despite saying tonight: "No, I'm not coming back, I'm retired. Not for ten million dollars."
The party's only MP refused to comment on this evening's news.
In the middle of supper. Not taking calls about UKIP stuff. It's shepherds pie, by the way— Douglas Carswell MP (@DouglasCarswell) October 4, 2016