Former Welsh Secretary Alun Cairns has become the third Tory MP to publicly switch their support from Rishi Sunak to Liz Truss in the Conservative leadership race.

Cairns said on Monday that he believes the Foreign Secretary is the best candidate to secure the Union of the four UK nations.

He effectively argued it would be more likely to break up under Mr Sunak, a former chancellor, compared to his opponent and her “positive ambition for our country”.

Ms Truss has adopted a more abrasive approach to the first ministers of the devolved administrations during the campaign to succeed Boris Johnson in No 10.

She has described Scotland’s Nicola Sturgeon as an “attention seeker” who is “best ignored” and called Wales’s Mark Drakeford a “low energy version of Jeremy Corbyn”.

Mr Cairns’ move comes ahead of Tory party hustings in Scotland and Northern Ireland this week, and as polls put Ms Truss the favourite to win.

Writing in The Mail+, Mr Cairns said: “At the outset of the campaign, I met Rishi Sunak to discuss the future of the Union and was content with his responses. I chose to support him.

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“However, as the campaign has developed, the risks to our Union have become more apparent. There is a need to break out of the current orthodoxy. The future of our Union depends on it.

“For this reason, I have concluded that with the positive ambition for our country and imagination she has shown, alongside her strong track record as a reformer, Liz Truss is best placed to secure our Union.”

Mr Cairns served in the Cabinet role under David Cameron, Theresa May and Boris Johnson, from 2016 until November 2019, when he resigned over his links to a Conservative candidate for the Welsh assembly who was accused of sabotaging a rape trial.

Current Welsh Secretary Sir Robert Buckland defected from the Sunak campaign to support Ms Truss on Friday, following former minister Chris Skidmore’s switch.

The contenders’ approach to securing the Union will come under growing scrutiny, with the pair heading to Perth on Tuesday and Belfast on Wednesday for questioning by the party members who will select the winner.