Former Turkish army chief Ilker Basbug, the highest-profile defendant convicted in a mass trial over an alleged coup plot, was set to walk free from jail on Friday, less than a year after being given a life sentence.

Former Turkish army chief Ilker Basbug, the highest-profile defendant convicted in a mass trial over an alleged coup plot, was set to walk free from jail on Friday, less than a year after being given a life sentence.

A staunch defender of the military in its showdown with the Islamic-rooted government of Prime Minister Recep Tayyip Erdogan, Basbug branded his trial a "black stain" on the country's history.

The 71-year-old, a former NATO-trained career soldier seen as a political moderate, once led Turkey's military campaign against outlawed Kurdish rebels but later took a conciliatory approach to resolving the three-decade conflict.

The decorated general was sentenced to life in prison last August after a long-running trial of 275 people accused of plotting to overthrow Erdogan's Justice and Development Party (AKP) government.

But on appeal, Turkey's top court ordered his immediate release, ruling that Basbug's rights were violated because a lower court had failed to publish a detailed verdict and send it to the appeals court.

Perhaps not coincidentally, the order for his immediate release came as an embattled Erdogan is seeking to improve ties with the military as he fights for his political survival in a feud with Muslim cleric Fethullah Gulen.

University professor Ahmet Insel, author of two books on the Turkish armed forces, said he does not count Basbug among the hardline generals who in 1997 helped bring down the government of the Islamist premier Necmettin Erbakan, Erdogan's mentor.

"He is a general who tries to keep the Turkish army in the barracks," Insel said, in a country where the military has carried out three coups since 1960.

He said Basbug was known to lean towards the main opposition Republican People's Party (CHP), although he has never voiced public support for the party created by Mustafa Kemal Ataturk, the founder of modern Turkey and the embodiment of the secular values the army defends.