Ahmed Hamdawi Oueid was sentenced to death by a Baghdad criminal court in May after being found guilty of killing Hashemi, an internationally recognised expert in Sunni Muslim extremism.

Hashemi, who was also a security adviser to the Iraqi government, was shot dead on July 6, 2020 by men on motorcycles in front of his home.

"All the decisions rendered by the central court are annulled... and the case is returned to the competent court responsible for investigating," the court of cassation said.

The ruling was issued on July 31 and published on the court's website this week.

Explaining its decision, the court said the committee responsible for investigating the killing had "no legal power" to do so.

The now-disbanded "Committee 29" had been set up by former prime minister Mustafa al-Kadhemi to fight corruption, among other issues.

In 2021, Kadhemi announced arrests and state television aired the "confessions" of Oueid, a 36-year-old policeman.

Hashemi's murder sparked outrage across Iraq and was denounced by Western countries as well as the United Nations.

Hashemi had thrown his support behind popular protests that had broken out in Iraq a year before his death against the ruling establishment, which was seen by many as inept, corrupt and too close to Iran.

More than 600 people were killed and thousands wounded in the protests that had erupted in October 2019 and the ensuing crackdown on the demonstrations.

In the aftermath of the protests, a spate of killings, attempted murders and abductions targeted dozens of activists in Iraq.