Diane James has quit as leader of the UK Independence Party after just 18 days in charge.
The 56-year-old was voted in as leader last month with 8,451 votes, having fought off competition from Lisa Duffy, Bill Etheridge, Elizabeth Jones and Phillip Broughton.
But in a statement on Tuesday evening, the 56-year-old MEP for South East England said that it had become clear in discussions with party officials that she did not have "sufficient authority" to push through changes which she had planned.
Ms James revealed she had not yet formalised her nomination as leader, meaning predecessor Nigel Farage is still technically in charge.
She said: "It is with great regret that I announce that I will not be formalising my recent nomination to become the new leader of the party with the Electoral Commission.
"Having won the enthusiastic support of party members, I was nominated by them as the new leader at the recent Ukip Bournemouth conference.
"Since that time, 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.
"For personal and professional reasons, therefore, I will not take the electoral process further."
The revelation that her nomination was not formalised has sparked debate that Nigel Farage would return, but he told the Press Association that he would not come back: "Not for ten million dollars."
Asked if he would take more money, he said: "No I'm not coming back, I'm retired."
According to one source within the party, many did not have confidence in James - they told ITV News correspondent Rupert Evelyn that she was an "accident waiting to happen":
The party's only MP, Douglas Carswell, tweeted to say that he would not take calls "about UKIP stuff" because he is "in the middle of supper".
Ukip now faces the prospect of a second leadership contest within the space of weeks, after a divisive race over the summer which exposed deep rifts between rival wings of the party.
Front-runner Steven Woolfe was barred from standing after submitting his nomination papers late, while Suzanne Evans was unable to take part as she was suspended.
Ms Evans confirmed her suspension from Ukip had been lifted, clearing the way for her to enter any leadership race.
But she declined to say whether she was considering throwing her hat into the ring, saying: "I think we all need to see what happens over the next few hours."
Ukip chairman Paul Oakden said: "It is with regret that I have tonight received confirmation that Diane James has chosen to resign as party leader, citing personal and other reasons. I will now look to convene an emergency meeting of our National Executive Committee to confirm the process for electing Diane's replacement.
"Whilst the decision is unfortunate, it is one that Diane is entitled to make. We thank her for all her work as leader, and as a hard working MEP, a role she will continue with her customary vigour."
Timeline: The brief rise and fall of Diane James as Ukip leader
June 23: Britain votes to leave the EU by a majority of 52%, prompting Nigel Farage to stand down as leader again
September 16: Deputy chairman Diane James elected to replace Mr Farage after securing 8,451 votes in a landslide victory
October 4: Ms James quits as Ukip leader after just 18 days in charge