Lena Baker, the only woman executed in Georgia's electric chair, was sentenced to death by an all-white, all-male jury after a trial that lasted just one day. In August 1944 Baker told the court that 67-year-old EB Knight, a man she had been hired to care for, had held her against her will in a grist mill and threatened to shoot her if she tried to leave.

She said she had grabbed Knight's gun and shot him when he raised a metal bar to strike her.

The decision to refuse Baker clemency in 1945, said a spokeswoman for Georgia Board of Pardons and Paroles, Scheree Lipscomb, "was a grievous error, as this case called out for mercy".

As Baker sat in the electric chair on March 5 1945, she said: "I am ready to meet my God." Moments earlier she had said: "What I done, I did in self-defence or I would have been killed myself. Where I was, I could not overcome it."

Her last words, along with her picture, are displayed near the now-retired electric chair at a museum at Georgia state prison in Reidsville.

Baker, a mother of three, often drank with Knight, and the two had a sexual relationship that sparked animosity in the small southern town of Cuthbert during the segregation era.

Knight's son, Eugene, testified he had warned Baker to stay away from his father. Later, he told the court he had found the two together, and: "I took her and beat her until I just did leave life in her."

Baker's body was buried in an unmarked grave behind a small church near Cuthbert, where she had been a choir member. In recent years her case has become a local cause celebre. The congregation of the church where she sang donated a concrete slab to mark her resting place. Since 2001 campaigners have commemorated the date of her execution at the graveside and surviving family members held a Mother's Day ceremony there in 2003.

Baker's grandnephew, Roosevelt Curry, who has led the family's efforts to clear her name, said he had cried when the board informed him of its decision. "Now we can all cry tears of joy," Mr Curry, 61, told the Atlanta Journal-Constitution. "She had nothing and no one stood by her. It's late but it's on time. This case was passed to me. I can pass this on to my family."

Mr Curry pursued his great-aunt's rehabilitation with the help of John Cole Vodicka, the director of the Americus-based Prison & Jail Project, a prison advocacy group.

"It's gratifying to see that this blatant instance of injustice has finally been recognised for what it was - a legal lynching," Mr Vodicka said.

Garland Hunt, the parole board's vice-chairman, said the board did not see the pardon as striking a blow against racial injustice or righting a historical wrong.

"We just felt that this was a situation that was unique," he said. "We felt it was a good thing to do for the family." "[We're] not saying she's innocent. As a matter of fact, the board does not find Lena Baker was innocent of this crime."

The pardon will suggest she "could have been charged with voluntary manslaughter" which carries an average sentence of 15 years in prison.