India's president Pratibha Patil has rejected the mercy petition of the three men convicted for the killing [EPA]

Three men convicted for1991 assassination of former Indian Prime Minister Rajiv Gandhi are to be hanged on September 9, officials have said.

The superintendent at Tamil Nadu's Vellore prison, where the men are being held, on Friday received the official communication to carry out the execution of Murugan, Santhan and Perarivalan.

"We have informed the convicts about the date of hanging. The execution will be (carried out) September 9 early morning," R Arivudainambi said.

On August 11, India's president Pratibha Patil rejected the mercy petition of the three men who were linked to the Liberation Tigers of Tamil Eelam (LTTE), which ordered Gandhi's killing.

A woman suicide bomber blew up Gandhi, who was prime minister of India from 1984 to 1989, at an election rally in Sriperumbudur near Chennai on May 21, 1991.

Murugan, Santhan and Perarivalan and others were charged with criminal conspiracy that led to Gandhi's killing.

In 1998, all 26 accused in the case - including the three - were sentenced to die by a special trial court.

In 1999, the Supreme Court confirmed the death sentences of four, Murugan, Santhan and Perarivalan and Nalini, an Indian Tamil woman married to Murugan who was also convicted in connection with the assassination.

The death sentence of Nalini was commuted to life imprisonment after her mercy petition was accepted.

The capital punishment of the others was reduced to varying terms of imprisonment.

In 2006, the Tamil Tigers expressed "regret" for the murder.

The death penalty is rare in India. The last execution was in 2004 when a 41-year-old former security guard was hanged for the rape and murder of a 14-year-old schoolgirl.