Rishi Sunak dealt big blow in bid to be UK PM, loses this minister’s support

Truss, who is on course to defeat former finance minister Rishi Sunak according to opinion polls of members of the ruling Conservative Party, was "authentic" and "straight", Wallace wrote in The Times.

British foreign minister Liz Truss received another boost for her campaign to become the country's next prime minister when defence secretary Ben Wallace, one of the most popular members of the government, gave her his support on Friday.

British Conservative Party member Rishi Sunak launches his campaign for the Conservative Party leadership, in London.  (AP)

Truss, who is on course to defeat former finance minister Rishi Sunak according to opinion polls of members of the ruling Conservative Party, was "authentic" and "straight", Wallace wrote in The Times.

"I have sat with her in cabinet, bilateral meetings and international summits. She stands her ground. Above all, she is straight and means what she says," he said.

Wallace also took a swipe at Sunak for quitting as Prime Minister Boris Johnson's chancellor of the exchequer in early July which helped trigger a wave of other resignations that culminated in Johnson announcing that he would stand down.

"I don't have the luxury as defence secretary of just walking out the door — I have roles in keeping this country safe," Wallace said in an interview with The Sun newspaper.

"And the guardian of the markets, you know, the guardian of our economy, is the chancellor."

The outcome of voting by Conservative Party members for their new leader will be announced on Sept. 5.

Wallace had been seen as the early favourite to replace Johnson as prime minister before he ruled himself out.