The Foreign Secretary, seen as the frontrunner in the race to replace Mr Johnson, has offered strong indications that she would lift the ban on fracking amid soaring energy bills and a cost-of-living crisis.
Mr Johnson used his final major policy speech, at the Sizewell nuclear plant site in Suffolk, to say he is “dubious” about the benefits that fracking could provide.
The PM, who promised £700 million of funding for the Sizewell C plant, also suggested that green energy is a much more viable option.
“I’m not intellectually, morally opposed to this at all,” he said.
Taking questions from reporters, he added: “I think that if we could frack effectively and cheaply in this country, that would possibly be a very beneficial thing.
“I just have to say I’m slightly dubious that it will prove to be a panacea. I would much rather that we focused on the things where we are brilliant and where the environmental damage is really minimal, like offshore wind.”
He stressed how cheap offshore wind is compared to gas, calling the UK “brilliant” at the technology.
Mr Johnson continued: “Of course we need to diversify policy. Where local communities want a different solution, they should be allowed to go for different solutions, but I don’t think that particular solution (fracking) is going to be the panacea that some people suggest.
“We can be flexible, but we should not put all our eggs in that particular basket.”
The energy crisis has dominated the latter stages of the Tory leadership contest, amid grim warnings about a long winter ahead for UK households and renewed debates about energy security in the wake of the war in Ukraine.
It was reported last month that Ms Truss would invite applications for drilling licences to explore new fields in the North Sea if she becomes prime minister, as well as push oil and gas firms to invest in their existing sites to maximise production.
Rishi Sunak, who is widely believed to be in for defeat on Monday, has also set out plans to boost North Sea gas production and to allow fracking where it is supported by locals.