Diane James has quit as leader of Ukip after just 18 days in charge, a senior party source has said.
Ms James was elected successor to Nigel Farage in a landslide victory on September 16.
Reports suggested that she had decided to step down because of her husband's ill health.
There was no immediate response from the party to reports of Ms James's resignation.
An MEP for South East England since 2014, 56-year-old Ms James was the Eurosceptic party's first female leader and won the post by a wide margin after winning the backing of senior Ukip figures including Mr Farage.
But questions were raised about her commitment to the post after she declined to take part in hustings debates around the country with rival candidates.
Reports of Ms James's abrupt departure sparked immediate speculation that Mr Farage might return to lead Ukip for a third time, after stints from 2006-09 and 2010-16.
But asked if he would consider returning as leader, Mr Farage told the Press Association: "Not for ten million dollars."
Asked if he would take 20 million dollars, he said: "No I'm not coming back, I'm retired."