"I don't believe in windfall taxes because they put off future investment," Truss said, when asked in a hustings event for party members if it was time for another windfall tax after Shell reported record profits on Thursday.
"Windfall taxes send the wrong message to the world, they don't send the message that Britain is open for business," she said at the event in Leeds in northern England.
Former finance minister Rishi Sunak, who is battling Truss for the leadership, announced an Energy Profits Levy (EPL) on oil and gas producers in May to raise 5 billion pounds ($6 billion) to support households struggling with the soaring cost of living.
"What we should be doing is encouraging Shell and other companies to invest in the United Kingdom, because we need to get our productivity up, we need capital investment," Truss said.
She leads in opinion polls among Conservative Party members, who will ultimately decide who becomes Britain's next prime minister on Sept. 5 after weeks of voting.
($1 = 0.8220 pounds)
(Reporting by James Davey, editing by Deepa Babington)
