China has issued a warning to Australia to stop interfering in the investigation of Chinese-Australian writer Yang Hengjun.

China has issued a stern warning to Australia not to intervene in the case of writer and pro-democracy activist Yang Hengjun, who was yesterday charged with espionage.

Dr Yang has been detained under mysterious circumstances since January and held without access to lawyers, and was formally arrested last week.

Chinese authorities have charged him with spying, telling the Australian Embassy that he is “suspected of committing espionage crimes”.

Espionage is punishable by death in China.

Overnight, Beijing made a clear statement to Canberra, saying it “must not intervene”, following remarks by Foreign Minister Marise Payne earlier that she was “very concerned and disappointed”.

China’s foreign ministry spokesman Geng Shuang said: “I would like to emphasise that China is a country ruled by law and the Australian side should earnestly respect China’s judicial sovereignty and must not intervene in any cases handled in China.”

Feng Chongyi, an associate professor at Sydney’s University of Technology and a close friend of Dr Yang, told news.com.au he was “furious” at the news of Dr Yang’s formal arrest.

“This is outrageous political persecution. I hope the international community will join hands to demand the release of Dr Yang,” Dr Feng said.

Ms Payne pledged that she would continue to “advocate strongly” for his release.

“Dr Yang has been held in Beijing in harsh conditions without charge for more than seven months. Since that time, China has not explained the reasons for Dr Yang’s detention, nor has it allowed him access to his lawyers or family visits,” Ms Payne said.

“I have discussed this twice with China’s Foreign Minister, State Councilor Wang Yi, and have written to him three times, stating my concerns, and those of the Australian government and people.

“We have serious concerns for Dr Yang’s welfare, and about the conditions under which he is being been held. We have expressed these in clear terms to the Chinese authorities.

“It is important, and we expect, that basic standards of justice and procedural fairness are met. I respectfully reiterate my previous requests that if Dr Yang is being held for his political beliefs, he should be released.

“We expect Dr Yang to be treated in accordance with international human rights law, including the Universal Declaration of Human Rights, with special attention to those provisions that prohibit torture and inhumane treatment, guard against arbitrary detention and that protect the right to freedom of thought, conscience and religion.”

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Dr Yang is a writer, democratic activist and former Chinese diplomat who was born in China, but became an Australian citizen in the early 2000s.

As a blogger, he has written thousands of articles promoting the rule of law, democracy and human rights and built up a large following in China.

He was taken into custody on January 19 after flying from New York, where he lives, to Guangzhou. China did not tell Australia he had been detained until January 23, three days later.

Speaking to news.com.au in March, Dr Feng revealed China had only admitted Dr Yang was in their custody after reports of his detention appeared in the media, leading Australia to ask about him.

“The Chinese authorities had not informed the Australian government until the media reports,” Dr Feng said.

“If there’s no media report, there’s nothing to lead to the government inquiring.”

China accused Dr Yang of “endangering national security”.

He was held without charge for almost eight months — until now.

Dr Yang was detained in what China calls “residential surveillance” at an undisclosed location in Beijing from January until July.

“We would describe it as home detention. As Dr Yang doesn’t have a home in Beijing, he is being held in a similar situation as opposed to being held in a prison,” then Defence Minister Christopher Pyne said in January.

Human rights organisations — and Dr Feng — believe China’s so-called “residential surveillance” system is more sinister than that.

“The authorities can detain someone for as long as six months without laying a formal charge. When they do not have enough evidence, they do that,” Dr Feng said.

He said Dr Yang could endure “round the clock interrogation” in an attempt to force a confession.

On July 19, Australia received formal notification from Chinese authorities that Dr Yang had been moved to criminal detention.

Australian consular officials have met with Dr Yang seven times since his detention began, and the Department of Foreign Affairs and Trade (DFAT) has repeatedly expressed concern that he has not been given access to proper legal representation.

“We have worked tirelessly and in good faith with the Chinese government to advocate for Dr Yang’s interests since he was detained. We expect basic standards of justice and procedural fairness to be met,” Ms Payne said in July.

Meanwhile Dr Yang’s supporters, Dr Feng foremost among them, have accused the Australian government of a “very, very weak” response.

“The Australian government has an obligation to ask for the immediate release of their citizen rather than saying in these very soft words that it’s a police matter under Chinese law,” Dr Feng said.

He said there should “absolutely” have been a stronger push for Dr Yang’s release, more along the lines of the successful international campaign to free Australian football player Hakeem Al-Araibi from prison in Thailand.

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Dr Feng believed the government’s subdued treatment of China could put other Australians at risk in the future.

“If you allow this to become a precedent, then the authorities can simply detain anyone for any political consideration,” he said.

The organisation Reporters Without Borders says there are more than 60 journalists and writers behind bars in China, including the award-winning photographer Lu Guang.

In its 2018 World Press Freedom Index, China ranked 176th out of 180 countries.