China's top court on Friday cleared a man executed 21 years ago for murder -- more than a decade after another man confessed to the killing -- in the latest miscarriage of justice in the Communist-ruled country.

Nie Shubin was 20 years old when he faced a firing squad in 1995, two days after being convicted of rape and murder.

"The Supreme People’s Court believes that the facts used in the original trial were unclear and the evidence insufficient, and so changes the original sentence to one of innocence,” it said in a statement on a verified social media account.

Chinese courts have a conviction rate of 99.92 percent, and concerns over wrongful verdicts are fuelled by police reliance on forced confessions and the lack of effective defence in criminal trials.

Overseas rights groups say China executes more people than any other country, but Beijing does not give figures on the death penalty, regarding the statistics as state secrets.

Nie was convicted of raping and murdering a woman whose body was discovered by her father in a corn field on the outskirts of Shijiazhuang city, in the northern province of Hebei.

But the time, method and motive for the murder could not be confirmed, and key documents related to witnesses and the defendant's testimony were missing, the supreme court said.

The "primary evidence was that Nie Shubin's confession of guilt corroborated the other evidence", but "there are doubts over the truth and legality of his confession of guilt", the statement added.

Nie's family had been campaigning for justice since a serial murderer arrested in 2005 confessed to the killing. But the case was only formally reopened in 2014.

"Thanks to all those who helped on Nie Shubin's case!" his mother, Zhang Huanzhi, 72, said on social media.

- 'Inquisition by torture' -

The Hebei high court, which convicted and executed Nie, "expressed deep, deep regrets" to his relatives and would investigate "possible illegal problems related to the trial" soon, according to state broadcaster CCTV.

But Liu Fujin, one of the defence lawyers involved, said the court had been unwilling to reconsider the case for years because no one would take responsibility for a mistaken verdict.

He had applied to see documents related to the case 54 times to no avail, he told AFP.

"So many examination statements and documents have been tampered with or lost; how could they let lawyers look at documents messed with to such an extent?" he said.

Though the verdict was a "landmark" and China's judicial system had improved since the 1990s, he said, many injustices remain unaddressed.

"Every province and every region has old cases just like Nie’s and new ones coming up that are all still being suppressed," he said.

"Trial dossiers and evidence in favour of defendants are still being concealed, and inquisition by torture is happening every day."

In 2014, an Inner Mongolian court declared innocent Hugjiltu, who had been executed at age 18 in 1996 for rape and murder -- only for another man to confess to the crime later. Last February, China "penalised" 27 officials involved, issuing them demerits.

William Nee, China researcher for Amnesty International, which campaigns against capital punishment, said: "There are many severe flaws in China's application of the death penalty, and the only way they can really ensure that this sort of tragic case doesn’t occur in the future is by abolishing it."