Two days before South Carolina is scheduled to execute a man on death row, the key witness for the prosecution has come forward to say that he lied at trial and the state was putting to death an innocent man.

Khalil Divine Black Sun Allah, 46, is due to be killed by lethal injection on Friday. His attorneys filed emergency motions to halt the execution, citing new testimony asserting that Allah was wrongfully convicted, but the state supreme court denied the requests late Thursday, saying the execution should proceed.

Allah, who was previously known as Freddie Owens, was convicted of an armed robbery in November 1997 and the murder of a convenience store cashier named Irene Graves. He was 19 years old at the time. Allah has long asserted his innocence in the killing of Graves, a 41-year-old mother of three, who was shot in the head during the robbery.

There was no forensic evidence tying Allah to the shooting. The state’s central evidence against Allah was testimony from his friend and co-defendant, Steven Golden, who was also charged in the robbery and murder. Golden and Allah were due to face a joint murder trial, but as the case was starting in 1999, Golden pleaded guilty to murder, armed robbery and criminal conspiracy and agreed to testify against Allah.

Surveillance footage at the store showed two masked men with guns, but they were not identifiable in the footage. Golden, who was 18 years old at the time of the robbery, said at the trial that he and Allah had been the men in the footage, and that it was Allah who shot Graves.

But on Wednesday, two days before the scheduled execution, Golden came forward with an explosive affidavit, stating that Allah “is not the person who shot Irene Graves” and “was not present” during the robbery. Golden said he concealed the identity of the “real shooter” out of fear that “his associates might kill me”, and that he was coming forward now because he wanted “a clear conscience”.

“I don’t want [Allah] to be executed for something he didn’t do,” he wrote in the new affidavit.

Golden said he was high when police questioned him days after the robbery, and the detectives claimed they knew Allah had been with him: “They told me I might as well make a statement against [Allah], because he already told his side to everyone and they were just trying to get my side of the story. I was scared that I would get the death penalty if I didn’t make a statement.”

He alleged prosecutors later promised him that he would not face the death penalty or life in prison if he testified against Allah, and he agreed.

In a motion filed with the court late on Wednesday, Allah’s lawyers said: “If this Court does not grant a stay, [Allah] will die for a crime that he did not commit.”

In a response filed on Thursday, the attorney general’s office suggested that Golden was not credible, saying he “has now made a sworn statement that is contrary to his multiple other sworn statements over twenty years”. Lawyers for the state also noted that people in Allah’s life at the time testified that Allah had confessed to them that he was the shooter. Allah’s attorneys rejected the allegations that he had previously confessed, suggesting that testimony was unreliable.

The state supreme court sided with the attorney general, ruling the new evidence did not amount to “exceptional circumstances” warranting a reprieve.

The justices said Golden’s new statement was “squarely inconsistent” with his repeated past testimony, and that there was “no indication of the circumstances under which Golden was asked to sign his most recent affidavit”. The justices also maintained that other evidence suggested Allah’s guilt.

Allah’s execution would be the first in 13 years in South Carolina and could be the start of a series of rapid executions in the coming months in the state. The state supreme court recently announced five additional executions that it would seek to schedule after Allah is killed, saying they would be spaced apart by at least 35 days.

South Carolina had unofficially paused executions in 2011 as pharmaceutical companies stopped supplying lethal injection drugs, fearing public pressure. But the state restocked its supply after it passed a law last year shielding the identity of suppliers.

Allah’s attorneys had raised a series of objections to his execution in recent weeks, before Golden’s new statement. They noted Allah had been convicted of murder without a jury explicitly ruling that he pulled the trigger. Prosecutors told jurors at his trial that they could convict him for murder simply if they believed he was present during the robbery, and his lawyers have argued that the death penalty should not be applied to a defendant found guilty as an “accomplice”.

His lawyers have also noted that he endured a lifetime of severe violence and trauma and had been diagnosed with brain damage.

If executed, Allah would be one of the youngest people at the time of the crime to be put to death by South Carolina in decades.

Allah’s lawyers have also filed a clemency petition with the governor’s office to stop the execution.

The Rev Hillary Taylor, executive director of South Carolinians for Alternatives to the Death Penalty, said on Thursday it would be a “huge miscarriage of justice” if the execution proceeds: “Khalil should not have to die for somebody else’s wrongdoing. That is not accountability. That is not justice.”