Nalini's death sentence was commuted to a life term
Indian authorities have rejected a request for early release made by the woman convicted over the assassination of Indian PM Rajiv Gandhi.
Nalini Sriharan, who is in a prison in Tamil Nadu state, says she has been eligible for early release since 2005.
Last year she held a fast in the prison in support of her demand.
Rajiv Gandhi was killed by a suicide bomb at an election rally in May 1991. India has always blamed the attack on Sri Lanka's Tamil Tiger rebels.
At least 14 other people - including the suicide bomber - were killed in the attack.
Sriharan was convicted for being part of the assassination squad and was one of four plotters initially given a death sentence.
This was commuted to a life term following a plea for clemency by Rajiv Gandhi's widow and Congress Party president, Sonia Gandhi.
Sriharan has served more than 18 years in Vellore prison. She claims she is only required to serve 14 years in prison before being entitled to early release under Indian laws.
The government of Tamil Nadu, however, rejected her request on Monday.
"Considering the social history, circumstances of criminal behaviour, degree of criminality, her case is not fit for recommendation on premature release," an advisory board appointed by the government to consider her plea said.
In 2006, the Tamil Tiger rebels expressed "regret" for the murder of Rajiv Gandhi in a move which correspondents say was a realisation by the rebels that the assassination was a huge mistake.
Sriharan also went on hunger strike in 2006 because she wanted her daughter, who lives in Sri Lanka, to be given a visa to visit her in India.
In 2008, Priyanka Gandhi, daughter of Rajiv Gandhi, met Sriharan in prison.
"I don't believe in anger or violence and I refuse to let it overpower me. Meeting Nalini [Sriharan] was my way of coming to terms with my father's death," Ms Gandhi said after the meeting.
Sri Lankan troops defeated the Tamil Tigers in 2009.