Ruth Davidson has said she is “not sure” Boris Johnson is “in a position to answer” questions about what would happen if the UK left the EU without a deal.
The Scottish Conservative leader was speaking as part of a Newsnight Special being broadcast from Edinburgh in a week where she threw her support behind rival prime minister candidate Jeremy Hunt.
Mr Johnson has been clear on his view Britain would leave the European Union “do or die” on October 31.
But Ms Davidson also said she did not think the deadline “should be written in stone”.
When asked what would happen if Mr Johnson became PM, she replied: “I think we would leave the European Union without a deal.
“In terms of the butterfly flapping its wings, and what are the consequences of that, I’m not sure I’m in a position to answer that and more importantly I’m not sure he is.
“For me it’s a deadline that was decided upon by people saying can we have a bit of an extension.
“There’s nothing that’s written in stone that says that if we’re really close to a deal, at one minute past midnight on the 31st, that actually we can’t do that deal on the 1st, we’ve got to then leave without a deal.”
Ms Davidson also criticised Mr Johnson’s proposed tax plans, which would increase the threshold where people pay a higher rate of income tax from £50,000 to £80,000.
She said: “I don’t think it’s designed to help the Conservative Party, or indeed to help the country because I think that as a party we work best when we’re making sure that we’ve got fair taxation aimed particularly at the lowest earners.
“And I’ve been very clear that I think if you’re going to have a taxation policy that’s involved in cutting taxes for workers you should absolutely focus on the poorest first, on the lowest earners first.
“I think we as a party until quite recently… had had an agreement about that.
“I don’t think that comes as a surprise to any of your viewers… I’m you know not from the same wing of the Conservative Party as Boris Johnson.”
Despite backing Mr Hunt in the race, the interview raised a disagreement between the pair over changes to abortion law.
The MSP, who became a mother last October, told Kirsty Wark she thought it was “a terrible policy” and does not think she has spoken directly to the Number 10 candidate since his remarks on the issue.
She added: “It’s also a devolved issue and there are no plans from any party in Scotland.
“I don’t know whether that’s a faith issue for him, because we’ve not had a discussion about it, it largely came out of nowhere and I haven’t had a fulsome conversation with him since.
“I don’t know what is behind his reason for that. I personally take a very different view and I’m happy to expand upon that.
“In terms of the science, in terms of the woman’s right to choose, in terms of having ownership over your own body, and that’s why, like I say it is a devolved issue.
“The Scottish Parliament decides that for people in Scotland, and that no party including my own would do anything to change the timescale for abortions in this country.”
The programme was recorded in Edinburgh to mark 20 years since the Scottish Parliament’s creation.
Speaking about the milestone, Ms Davidson said: “I think actually that parliament has become the centre of Scottish civic and political life and I think when it first was established I was a student.
“I remember sitting in my student union here in Edinburgh and hearing Concorde flying over my head while watching the pictures of Concorde flying over my head on the television.
“And I wasn’t sure then what it would look like, what it would sound like, what it would mean.
“But it really has replaced Westminster as where the reporting comes from, where the policies come from.
“People think of this now as the place that runs Scotland.”