Chennai, Sep 21 - S. Nalini, undergoing life imprisonment in the Vellore jail for her involvement in the assassination of former Indian prime minister Rajiv Gandhi, began an indefinite fast Monday to demand her immediate release, officials said.

Nalini, 43, said she would continue her protest till she is released from prison.

According to the prison authorities, Nalini informed them about her decision to go on fast Sunday but gave it in writing only Monday morning.

Superintendent of Police, Vellore prison, Jeyabharathi told IANS over phone that the prison authorities have informed the state government, Vellore collector and other officials about Nalini's protest.

A doctor checked Nalini's health Monday and her health condition will be monitored daily, she added.

Nalini was awarded death penalty in 1991 but later at the intervention of Congress chief -- and Rajiv Gandhi's widow -- Sonia Gandhi, the death sentence was commuted to life imprisonment.

Last week, Nalini had filed a petition in the Madras High Court asking for the Tamil Nadu government to convene the advisory board to consider her case for release from the jail.