Sriharan is currently lodged in the Vellore Central Prison in Tamil Nadu. Three other convicts Robert Pious, Jayakumar and Ravichandran are undergoing life sentence for their role in the assassination carried out on May 21, 1991 in Sriperumbudur.
Earlier this year, the SC commuted the death sentence of the co-accused Sri Lankan nationals Murugan and Santhan and of AG Perarivalan, the Indian connection in the case.
Their sentences were commuted on the grounds that there had been a delay in the Centre deciding their mercy plea.
The release of four others, who had been given life sentences for the assassination, was also announced by the Tamil Nadu government led by former chief minister Jayalalithaa, citing provisions of the Code of Criminal Procedure.
However, the Supreme Court rejected their release and constituted a Constitutional Bench to look into the matter.
The former Prime Minister's assassins were convicted under the Terrorist and Disruptive Activities (Prevention) Act in January 1998. Three of the accused had been awarded the death sentence and four had been awarded life sentences.
Rajiv Gandhi was assassinated in a suicide bomb attack in Sriperumbedur, Tamil Nadu, on May 21, 1991.
Nalini's counsel Duraisamy had claimed that she has been languishing in the prison for nearly 18 years 'for no fault of hers'.
He further claimed that she had not committed any offence, except accompanying someone to the election rally, in which former prime minister Rajiv Gandhi was assassinated.
A New Yorker's Diwali in Delhi
Sri Lanka ODIs: Dhawal Kulkarni replaces Mohammed Shami
PIX: Deepika, Hrithik at Aamir Khan's Diwali bash
What's so special about these two Kawasaki bikes?
Black money case: Gold trader denies having Swiss bank account