Kemi Badenoch has branded the prospect of an electoral deal with Nigel Farage “for the birds”.
The Tory leader, who has been in post for just over 100 days, warned a pact with Reform UK could lead to more voters deserting the party.
Mr Farage’s party has been boosted by an array of opinion polls which shows Reform top, with an average of 25 per cent of the vote.
Former Tory Cabinet ministers Sir Brandon Lewis and Esther McVey suggested Mrs Badenoch should work with Reform to avoid an electoral wipe out.
Many on the right of British politics fear Reform UK and the Conservatives could split the vote, leading to Labour winning again in 2029.
But the Conservative leader declared: “The Conservative Party is a broad church. When we had disagreements, what people saw was disunity. We’ve now got a place where we are unified.
“The idea that you just do something with a whole different bunch of people and it’s going to be fine is for the birds. Politics just doesn’t work like that.
“There are many people who vote Conservative, who, if they think that we’re having mergers or pacts or whatever with Reform, will go elsewhere.”
At a farming rally in Westminster on Monday, Mr Farage said his party would not enter a pact with the Tories.
He said: “To make a pact with people, you’ve got to think, ‘I’m going to shake your hand and you’re an honourable person’.
“After the betrayal post the 2019 election, we do not believe them to be honourable. Simple as that, so the answer is no.”
Mrs Badenoch was asked whether she could foresee “any circumstances” in which she would strike a pact with Mr Farage.
She responded: “No, not me. Nigel Farage has said that he wants to destroy the Conservative Party.
“I have been given something very precious. I am the custodian of an institution that has existed for nigh-on 200 years. We have no guaranteed right to exist. There is no guarantee that we will be in government. But I have to look after this thing. I can’t just treat it like it’s a toy and have pacts and mergers.”
She described how Conservative supporters on the doorstep had recently told her: “If you get into bed with Farage, heaven forbid the image, we are done with your party.”
Reform UK topped a YouGov voting intention poll for the first time last week. The poll showed that if a general election were held tomorrow 25 per cent of British voters would choose Reform, 24 per cent would pick Labour, and 21 per cent would vote for the Conservatives.
The poll also shows that only 60 per cent of people who voted Labour in July would do so again now, and one in four Conservative voters would now vote Reform.
Around 43 per cent of Tory voters were in favour of the two parties merging. Reform UK chairman Zia Yusuf said if there was a general election tomorrow, the polling suggests Reform UK would win between 140 and 200 seats in the House for Commons.
He claimed Reform UK would win up to 400 seats at the next General Election.