LONDON—The former work supervisor of Andrea Leadsom, one of the two candidates to be the next U.K. prime minister, has dismissed a controversy about how she presented her career and described her as “totally honest.”

Ms. Leadsom has faced repeated questions about her résumé since the energy minister entered the race to lead the governing Conservative Party following Prime Minister David Cameron’s resignation.

Bob Yerbury, who as chief investment officer at fund manager Invesco Perpetual was Ms. Leadsom’s manager for 10 years, said she would make an “outstanding” prime minister, with “great conviction, great energy.”

“I would never, ever doubt her honesty,” he said. “Andrea doesn’t play games.”

However, he said that she didn't run a fund or make investment decisions, contrary to the claims made by one member of Parliament, Bernard Jenkin, who supports her.