Ruth Davidson has accused Boris Johnson of putting Brexit ahead of preserving the Union as she expressed support for Jeremy Hunt in the race to Number 10.

The Scottish Conservative Party leader's attack came as the first ministers of Scotland and Wales, the SNP’s Nicola Sturgeon and Labour’s Mark Drakeford, joined forces to demand that the next Prime Minister rules out a “disastrous” no-deal.

Referring to Mr Johnson’s unequivocal vow to deliver Brexit “do or die” by October 31, she said: “I want to see him make assurances that it’s not Brexit do or die, it’s the Union do or die.

READ MORE: Boris Johnson: 'I have a very good relationship with Ruth Davidson'

“That’s exactly what we’ve seen from the other candidate in the race and that’s why he’s going to get my vote.”

When Mr Hunt was asked earlier this week what would come first out of Brexit and the Union, he responded: “The Union, every time.”

The foreign secretary is the third candidate to have been endorsed by Ms Davidson in the Conservative leadership race, who has previously supported Sajid Javid and Michael Gove.

Mr Hunt is going head-to-head with Boris Johnson to become the next Tory leader and prime minister.

READ MORE: Ruth Davidson backs Jeremy Hunt for PM

The winner of the contest, which will be decided by a vote of about 160,000 Conservative Party members, will take over from Theresa May on 24 July.

Ms Davidson tweeted that "any Conservative leadership candidate must put the Union first. Jeremy has done so and will get my vote".

Ms Davidson has been a fierce critic of Mr Johnson in the past, famously clashing with him over Brexit during a TV debate ahead of the EU referendum.

But Mr Johnson has the support of at least four Scottish Conservative MPs, including Ross Thomson - who was appointed earlier this week to run Mr Johnson's campaign in Scotland.