Nicola Sturgeon said the choice of having either Boris Johnson or Jeremy Hunt as the next Prime Minister was like “asking me if I'd prefer to be run down by a lorry or a bus”.
The SNP leader has refused to endorse either candidate, but had particularly harsh words for the frontrunner.
“I don't endorse Jeremy Hunt - but I look at Boris Johnson right now, and I find it really difficult to get my head round how any rational person could seriously contemplate putting him into the highest political office in the UK,” she said.
“And the fact that the Tories appear very seriously to be contemplating doing that I think gives you a sense of how out of kilter the whole UK political system has become and how out of touch the Tories are with mainstream opinion in Scotland.”
She has thrown her support behind fellow SNP MP Ian Blackford, who last week called Mr Johnson a “racist” who is “not fit for office”.
“I don't know what's in Boris Johnson's head when he makes the overtly racist comments that he makes so I don't know whether he's just doing it for headlines or whether it reflects actual sentiment on his part,” she said.
“But if you make overtly racist comments like the comments he made about Muslim women, you can't then throw your hands up in horror when people call you a racist, because that looks as if that's what you are.”
Ms Sturgeon has reiterated her previous comments on Scotland leaving the UK and joining the European Union.
She said: “if Scotland chooses independence and wants to be in the EU, we will be in the EU.”
“It is inconceivable that the EU is going to stop a pro-European independent Scotland becoming part of it.”