Cambodia's ​former opposition leader Kem Sokha, who was serving a 27-year sentence for treason, has been pardoned, the country's former prime minister said.

Hun Sen, ​who is currently Cambodia's acting head of state, signed a decree pardoning his political rival on behalf of King Norodom Sihamoni.

Sokha, the former leader of the now-dissolved Cambodian National Rescue Party (CNRP), was first arrested in 2017 over a video where he said he had received support from US pro-democracy groups.

He has been held under house arrest since he was found guilty of treason in 2023. The charges have been widely derided as politically motivated by human rights groups.

Hun Sen posted on Facebook that Sokha had been "pardoned", alongside a photo of the royal decree signed by him.

His son, Hun Manet, who took over as prime minister from his father in 2023, said the pardon was "one more step towards strengthening national unity".

It came after an appeal against Sokha's sentence was rejected last month. But it did not include overturning a ban on the politician leaving Cambodia for five years.

Elaine Pearson, Asia director at Human Rights Watch, said: "Hun Sen's decision to pardon Kem Sokha after more than eight years in arbitrary detention partially reverses a grievous injustice, but it is deplorable that Sokha remains barred from participating in politics or leaving the country.

"Cambodia's remaining opposition politicians and parties are still under constant threat of arbitrary arrest and baseless restrictions. The government needs to ensure that political rights are respected in the country."

Sokha's CNRP party came close to securing a shock victory in the 2013 general election over Hun Sen's Cambodian People's Party (CPP) despite accusations of vote-rigging and intimidation.

As the next election approached, Sokha's party was the only viable threat to Hun Sen's rule.

The opposition leader was arrested in 2017, less than a year ahead of the crucial vote, which the CNRP was eventually banned from contesting, making Cambodia a de facto one-party state.

His arrest coincided with a crackdown on government critics.

On the day of his arrest, the Cambodia Daily published its last ever newspaper as it was shut down amid a crackdown on independent media. It ran a front page headline "Descent Into Outright Dictatorship" above a photo of a startled Sokha in handcuffs.

In 2018, his daughter Kem Monovithya told the BBC the only chance of her father being released was if his detention became a genuine burden to Hun Sen: "If there's no cost of keeping him he will continue to keep him."

The US embassy said at the time that the Sokha case had been "based on a fabricated conspiracy" and the conviction was a "miscarriage of justice".

Hun Sen, who ruled Cambodia for nearly four decades, has been accused of weaponising the country's courts to target his opponents. He stepped down as prime minister in 2023 and handed power to his eldest son, Hun Manet.

However, Hun Sen still wields immense power in Cambodia and is acting head of state while King Norodom Sihamoni receives medical treatment abroad.