UK Independence Party leader Lord Pearson of Rannoch is to stand down after less than a year in the position.
In his resignation statement, Lord Pearson said he was "not much good" at party politics and UKIP "deserved a better politician... to lead it".
The outgoing leader had a difficult general election campaign, telling one interviewer he could not remember his own manifesto in detail.
An interim leader will be chosen at UKIP's annual conference next month.
One of UKIP's two deputy leaders - Euro MP David Campbell Bannerman - is planning to stand in the contest to replace Lord Pearson, a spokesman said.
Lord Pearson's predecessor, Nigel Farage, said he would not announce whether he would stand until the conference.
Lord Pearson was elected in November 2009, after Mr Farage stepped down to concentrate on contesting the Buckingham parliamentary seat, which was held at the general election by Commons Speaker John Bercow.
Resigning as leader, Lord Pearson, 68, said he wanted to "spend more time on his wider interests" which included "the treatment of people with intellectual impairment, teacher training, the threat from Islamism and the relationship between good and evil" - as well as his dogs and family.
UKIP had increased its vote by 50% in the general election and "had many exciting plans for the future", he said, but it was time for a "younger leader" to show how "liberating and enriching life would be outside the EU", saying he was "confident" one would emerge.
He said the party had "never been more important for our freedom as a self-governing democracy" and had to "go on telling the truth about Europe".
"We have a coalition government which supports every new power grab by Brussels: supervision of our financial services; an EU diplomatic corps; new police and surveillance powers; bailing out the folly that is the euro.
"Much of this is illegal under the Treaties, but that has never worried Brussels or the Luxembourg Court, which now make most of our national law in a secretive process over which Parliament has no control.
"History teaches us that trouble lies ahead when a regime is free to break its own laws with impunity, when it is supported by a puppet court, and when its people are powerless to get rid of it. That is what the European Union has become, and the only way out is the door," he said.
Mr Farage, who was hurt in a plane crash on polling day in May, said he had yet to decide whether to stand for re-election to his old job.
He told the BBC: "I'm not going to say I'm absolutely not going to do the job again but I've got to decide, in the wake of that accident, whether I'm strong enough to take the job on.
"The other problem is I'm still leading a group in the European Parliament in Brussels. Can I do that and lead a party in the UK?"
He added: "I'm sad that he [Lord Pearson] is going... I don't think today's the day to announce whether one intends or not to run."