Diane James has quit as leader of Ukip after just 18 days in charge.
The 56-year-old said it had become clear in discussions with party officials that she did not have "sufficient authority" to push through changes which she had planned.
Ukip party chairman Paul Oakden said that Nigel Farage is "very unlikely" to return as leader but added: "I wouldn't say anything is impossible right now."
Speaking to BBC Radio 4's Today programme, he said Farage could still be listed as the registered leader of the party if the Electoral Commission has not received paperwork from Diane James following her election win.
He said: "If they still have Nigel Farage down as the leader it's going to make it a very interesting day for me, I suspect."
Oakden said he had not spoken to James about her decision to resign after 18 days in the role, adding: "I can only speak for my part and the part of the NEC which is they were as constructive as they could be under the circumstances of a new leader being elected."
Diane James has quit as Ukip leader before most people knew who she was. What happened?
Bill Etheridge, who stood against Diane James for leadership, said he was disappointed at her resignation adding she was supported by MEPs.
She has been in the role less than three weeks.