Known for her in-depth and often critical reportage of Chinese elite politics over her decades-long career, Gao Yu, 71, was handed her sentence in a closed-door hearing held under tight security in Beijing's No. 3 Intermediate People's Court on Friday.

She told the court she would immediately appeal the sentence, her lawyer Shang Baojun said.

"We are very disappointed in the verdict, we feel it is unjust," Mr Shang told Fairfax Media. "We will appeal."

Ms Gao was detained by police in Beijing in April last year before the politically-charged 25th anniversary of the violent suppression of pro-democracy demonstrations in Tiananmen Square in June 1989.

While much of the detail around Ms Gao's case has been shrouded in secrecy, accusations made public by China's state-run media indicated Ms Gao had been indicted for providing an overseas Chinese news outlet, Mingjing, with a Communist Party directive, called "Document No. 9" which warned against the infiltration of Western ideals, including freedom of media and expression, constitutional democracy and universal human rights, into China.

In May last year, the septuagenarian was paraded on state broadcaster CCTV confessing to sharing a "secret document" with an overseas news website.

But her lawyers say her confession was obtained under duress, with authorities threatening the welfare of her adult son. Ho Pin, the editor-in-chief of Mingjing, raised similar concerns and publicly denied Ms Gao had leaked any secret documents to his publication.

Ms Gao had faced trial in November but courts had twice postponed its verdict. Mo Shaoping, another of Ms Gao's lawyers, said the delay was unusual and showed the courts did not have enough evidence to reach a guilty verdict.

Mr Mo said at the heart of the case was the issue of freedom expression and the definition of what constitutes a state secret.

"Who has the duty to protect a state secret?" Mr Mo told Fairfax Media before the verdict. "Should it be down to an average citizen – how did they know about the 'state secret' in the first place?"

Ms Gao had been imprisoned twice before. She was detained for more than a year after the violent suppression of pro-democracy demonstrations in Beijing in June 1989, and was later jailed for six years in 1993 for leaking state secrets.

William Nee, China researcher at Amnesty International, said Ms Gao ​was imprisoned solely for challenging the views of the government and should be released immediately.

"This deplorable sentence against Gao Yu is nothing more than blatant political persecution by the Chinese authorities," he said.

Maya Wang, of Human Rights Watch, said it was hard to see Ms Gao's prosecution over her "rigorous, critical reporting about vital government policies" as anything other than the latest effort by the Chinese leadership to silence virtually all debate.

"No 'China Dream' can be realised until peaceful criticism is respected," Ms Wang said, invoking the upbeat Communist Party slogan popularised by President Xi Jinping.

In a twist of fate, Ms Gao met Fairfax Media on the day of her disappearance. In what would prove her last interview before her arrest, she said she was pessimistic that the international attention around the Tiananmen anniversary would prove a catalyst for a greater push toward democracy.

"I think [people in] China are waiting for an opportunity and the leadership is carefully guarding against this," she said.

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