Kemi Badenoch has described the prospect of a coalition government with Reform UK as 'monstrous'.
The Tory leader said defectors who left her party for Nigel Farage's are 'some of the most dishonest people' she has met.
She insisted that if there was a hung Parliament at the next general election, she would rather go back to the country and have another vote than form a coalition with Reform.
Speaking at the Edinburgh Fringe Festival, she said: 'Reform has people who left the Conservative Party, some of the most dishonest people I have ever met in my entire life.'
Defectors to Reform include Robert Jenrick, Suella Braverman and Danny Kruger.
She also warned 'the country is going to be in a pretty bad place' if the Conservatives fail to form the next government – but insisted that she wanted to 'win on my own terms'.
Asked by host Iain Dale if not working with Reform could mean a possible Labour/Liberal Democrat coalition, Mrs Badenoch said: 'If we get a hung Parliament and people see that the parties can't form a government, you have another election.'
She insisted: 'What I want is a Conservative government, not a monstrous Reform/Conservative hybrid.'
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Hitting out at Reform's leader, Mrs Badenoch said: 'Nigel Farage dictates what he wants depending on what he had for breakfast that day.
'They are all over the place, they announce one policy, they forget it the next day.
'How do I work with people like that when I am making carefully curated plans and taking a systems-based approach?
'How do you work with a man who has fallen out with every single person he has ever had to work with in every single party that he has started? I have to win on my own terms.'