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