Former Tory leadership contender Tom Tugendhat has endorsed Liz Truss in the race to be prime minister, in a huge blow to Rishi Sunak.

Writing in The Times, Mr Tugendhat said that the foreign secretary’s promises of tax cuts are based on “true Conservative principles” and that she can unite the party.

He said: “Liz has always stood up for British values at home, and abroad. With her at the helm, I have no doubt that we will move with determination to make this country safer and more secure.”

Mr Tugendhat said that after watching the candidates go head-to-head in live TV debates, “only one has convinced me she’s ready”.

The chairman of the foreign affairs select committee said both contenders have “huge qualities and many talents” but that Ms Truss has an advantage on the world stage because of her cabinet position.

He said the world “is getting more challenging” as fuel prices rise across the globe, adding: “We need a leader with the foreign-affairs experience to build alliances and keep our country safe.”

He said: “That’s where allies come in. As foreign secretary Liz is starting with a huge advantage. She can make our voice count.”

Mr Tugendhat has become the latest high-profile Tory to back Ms Truss.

Defence Secretary Ben Wallace gave his backing to Ms Truss last night, describing her as “authentic, honest and experienced”.

Mr Sunak was initially the frontrunner in the race, winning the most support from MPs in the first few rounds of voting.

But polling has shown Ms Truss is more popular among Tory party members, who will ultimately decide who will be the leader and next prime minister.