An Australian writer and businessman who has been detained in China since 2019 has been declared guilty of espionage and was given a death sentence with two years’ probation on Monday, in a blow to warming relations between Australia and China.

The severe punishment for the businessman, Yang Hengjun, was first revealed by the Australian government, and then confirmed by China’s Ministry of Foreign Affairs in a daily briefing in Beijing. If Mr. Yang does not commit any crimes in those probationary two years, the sentence can be commuted to life imprisonment, Penny Wong, the Australian foreign minister, said in a statement. She described the verdict as “harrowing.”

The long detention of Mr. Yang — who is also known by his legal name, Yang Jun — has been one of the sources of tensions between Australia and China. Now the severe sentence may again weigh on relations, which had been improving after the election of a new, center-left Labor government in Australia in 2022. The prime minister, Anthony Albanese, visited Beijing late last year and has pressed for Mr. Yang’s release.

“The Australian government will be communicating our response in the strongest terms,” Ms. Wong said, adding: “We have consistently called for basic standards of justice, procedural fairness and humane treatment for Dr. Yang, in accordance with international norms and China’s legal obligations.” She said she had directed officials to call in Xiao Qian, China’s ambassador to Australia.

Mr. Yang, 58, was born in China and became an Australian citizen in 2000, completing a dissertation there that focused on the internet and democratization in China. Mr. Yang described himself as a former employee of the Chinese foreign ministry, and also wrote a trilogy of novels about China’s espionage apparatus. He had been critical of human rights abuses under the Chinese government, but became more cautious in his public comments in the years before his detention, when dissent in China came under tighter control.