Civil war appears to have broken out in Ukip as the party's election campaign chief launched a scathing attack on Nigel Farage, calling him a "snarling, thin-skinned, aggressive".
Economics spokesman told The Times Farage risked turning the party into a "personality cult".
O'Flynn, an MEP, added that Farage's behaviour also risked the party being seen as an "absolute monarchy".
O'Flynn's attack comes after Farage stood down for failing to win the Thanet South seat in the general election, but was reinstated three days later when the party refused to accept his resignation.
The economics spokesman said Farage had moved away from being a "cheerful, ebullient... daring" politician.
He also attacked senior aides within the party and told The Times "aggressive" and "inexperienced" staffers must be cleared out.
Another senior official, who was prominent during the election campaign, told the BBC they believed Farage's chief of staff and one other adviser were "out of control" and "dragging Nigel down".
The attack comes amid a separate row after Ukip's only remaining MP, Douglas Carswell, resisted pressure from the party to claim £650,000 of taxpayers' money to fund up to 15 additional members of staff.
In a meeting with Farage, the Clacton MP refused to budge from his position on the funds, saying it would be "completely inappropriate" to take the money.