Ukip's only MP has issued a fresh call for party leader Nigel Farage to stand down in favour of someone who radiated "sunshine and optimism".
Douglas Carswell said the party needed a "fresh face" at the top and questioned Mr Farage's response to Ukip's poor showing in the Oldham West and Royton by-election.
It is the latest sign of unrest at the top of Ukip, following Mr Carswell's suggestion that Mr Farage should "take a break" following the party's failure to make inroads at Westminster in the general election in May.
In an interview with BBC Essex, Clacton MP Mr Carswell said: "The Oldham by-election to me said very clearly that I think we need a fresh face."
Ukip failed to make widely-predicted advances in the Oldham poll, which saw Labour hold the seat with a majority of more than 10,000, with Mr Farage claiming the process was "bent" and complaining about the postal vote system.
Explaining his desire for change at the top of his party, Mr Carswell said: "I don't want to wake up the morning after the European referendum and hear people saying it was the postal votes."
Mr Carswell ruled himself out of running for the leadership, saying he did not have the "patience", but insisted he remained loyal to the party.
"I am 100% Ukip and I'm very committed to Ukip, I'm not going anywhere," he said.
But Mr Farage reportedly told the BBC that the MP should "put up or shut up".
Mr Carswell suggested that Mr Farage had taken the party as far as he could and Ukip needed to "change gear".
He said: "We all need to think very carefully as to whether or not we can build beyond the base we have now got without that change.
"We have gone from being a party with 2%-3% market share to a party with 13%, 14%, 15% market share, and that's a phenomenal achievement , an incredible achievement and no one can ever say that that hasn't been an achievement.
"But sometimes a start-up needs to change gear and to change its management if it is to go to the next level."
He said the party should avoid being seen as "unpleasant" and "socially illiberal".
Instead, Mr Carswell suggested Ukip would make a breakthrough in the polls if it was an "optimistic, sunshine, smiley, socially liberal, unapologetically free market party".