Ukip deputy chairman Diane James has won the contest to replace Nigel Farage as the party's leader.
The MEP, who was the frontrunner in the contest, took the top job with 8,451 votes.
The new leader said the party had "moved mountains on the political landscape" and was the "change movement of the United Kingdom".
She also said the Government was stealing her party's ideas, calling the prime minister "Magpie May".
Some government ministers are "still fighting the referendum", despite the British people's vote in favour of Brexit, Nigel Farage has said.
Speaking at his party's annual conference in Bournemouth, the prime minister was rowing back on her post-referendum commitments and urged party members to ensure the government delivered a "hard Brexit".
"We can be very proud of the fact that we won the war," he said, in a reference to the referendum result. "But we now must win the peace and the only mechanism to put pressure on the government ... is for Ukip to be healthy and for Ukip to be strong."
Farage's comments serve as a rallying cry toUkip ahead of its leadership election, which has seen the party riven by internal divisions.
Douglas Carswell has insisted he will "absolutely not" quit Ukip after Nigel Farage said he "really isn't doing anything for us".
With the election of Diane James, Ukip has tipped the gender balance in British politics, but true diversity is still some way off.
The grammar-educated former healthcare sector worker made her political name in defeat and holds Vladimir Putin up as a leadership mentor.