British Prime Minister Theresa May's Conservatives "cannot be trusted on Brexit," Diane James, the newly elected leader of the eurosceptic UK Independence Party, said on Friday.
"My view is, the Tory [Conservative] Party cannot be trusted on Brexit," said James, 56, who added that UKIP must ensure that May pulls Britain out of the European single market.
"The people voted for a globally out-looking Britain. They voted to control our borders," she told delegates at UKIP's annual conference following her appointment to succeed outgoing Nigel Farage.
Farage was one of the most prominent campaigners for Britain to leave the European Union ahead of a referendum on June 23 which delivered a shock result in favour of the so-called Brexit.
James promised to make UKIP push the government to achieve a "true, 100-percent European Union exit" amid rumours of the possibility of a so-called "Brexit light" deal with the bloc.
James warned May's government that it faces a "great political battle" if it tries to agree a "soft Brexit" that keeps Britain in the single market.
James also accused "magpie May," who is seen as more right-wing than her predecessor, David Cameron, of "stealing" UKIP policy ideas in education, defence and other fields.
She said her priority will be to prepare UKIP for fighting in the next general election, expected in 2020, referring to the party as "the opposition in waiting."
UKIP came third in a general election in May 2015, winning 13 per cent of the votes but only one seat in parliament under Britain's constituency-based system.
James took her election on Friday as an opportunity to slam the "flawed, morally bankrupt" electoral system, which is often criticized as unfair by smaller parties and independents.
Farage resigned in July following UKIP's success in persuading voters to opt for Britain's departure from the EU. He had led a campaign dominated by concerns over migration and impassioned speeches on claiming back British sovereignty.
After leading the right-wing party for a total of 10 years, he said he wanted to focus on his personal life because the vote for Brexit "means that my political ambition has been achieved."
Farage said he was "right behind" James and urged all UKIP members to support her.
Arron Banks, UKIP's biggest financial backer, said he was "absolutely thrilled" with James's election, saying she was a "pretty steely individual" capable of uniting the party.
"Brilliant result, and the only person who could step into Nigel shoes," Banks wrote on Twitter.
James easily beat four other candidates but the leadership election was marred by several disputes - notably the exclusion of Steven Woolfe, a protege of Farage, because his candidacy was submitted online a few minutes after the deadline.
James has been a member of the European Parliament since 2014, representing the constituency of South-east England, and is the UKIP spokeswoman for justice and home affairs.