Kemi Badenoch has attacked Nigel Farage over the £5m gift he received before the general election as she ruled out an electoral pact with Reform UK.

The Tory leader questioned Farage’s acceptance of the gift from the Thailand-based crypto billionaire Christopher Harborne in the months before he stood to become an MP in 2024.

Writing in the Mail on Sunday after Reform’s poor showing in Thursday’s byelections, she said: “The results have left the idea of the Conservatives doing a deal with Reform stone-dead.”

Badenoch appeared to dismiss Farage’s account of the £5m gift, which was first revealed by the Guardian in April. Since then, Farage has changed his story over the reasons for receiving the gift. At first, he said it was to pay for personal security for his lifetime. As he faced questions about this explanation, Farage changed tack, saying he considered it a reward from Harborne for having campaigned for Brexit.

Badenoch said: “Character matters in politics because character is what is tested when things get tough. Do you have the strength to face down your own backbenchers on wasteful welfare spending? Keir Starmer did not. He failed that test.

“And what kind of character accepts £5m in cash and says it was merely a gift? Mail on Sunday readers have far too much common sense to believe that. Nobody gets £5m in their pocket for nothing, whatever Nigel Farage claims.”

The gift is under investigation by the parliamentary standards commissioner, who will examine whether or not it ought to have been declared.

In Thursday’s byelections, Andy Burnham won Makerfield with a 9,000-plus majority over Reform’s Robert Kenyon and more than 50% of the vote. That result has paved the way for a Labour leadership challenge, with Keir Starmer expected to announce his departure as prime minister on Monday. Meanwhile in Aberdeen South, the Scottish Tories defeated the Scottish National party while Reform came a distant third.

Badenoch said the results showed why she was right to rule out a pact with Reform, who she said “dress like Thatcherites but act like Corbynites”.

She wrote: “When those elections were called, I came under intense pressure to ‘unite the right’. The argument sounded clever: the Conservative party had never won in Makerfield, so why not do a deal with Nigel Farage and stand down? Reform UK could do the same in Aberdeen and everyone would claim victory.

“Thankfully, I know terrible advice when I hear it and the results on Friday proved my point. Had I listened, our emphatic victory in Aberdeen would have been diminished. People would have sneered that we won only because Reform stepped aside. A win ‘helped’ by Reform would have been no real win at all.”

She stressed the differences between the Conservatives and Reform, writing: “We are not the same, and voters are not ours to trade like football cards.”

A spokesperson for Reform told the Guardian: “We won’t need to ever deal with the Tories. They broke Britain and we won’t give them a chance to do it again. We have now led the national opinion polls for well over a year.”