Indian President Pratibha Patil has rejected mercy pleas from three Tamils convicted of the 1991 assassination of former Prime Minister Rajiv Gandhi.
In theory, the ruling paves the way for their execution, officials say.
However, there are a growing number of people on death row who have exhausted all legal appeals but whose sentences have not been carried out.
Correspondents say bureaucratic delays and a shortage of hangmen have contributed to the backlog.
"The rejection [of the clemency petitions] happened last week after the president returned from a foreign tour," presidential spokeswoman Archana Datta told AFP news agency.
The appeal was filed to the president by three Tamil men convicted for the assassination - Murugan, Santhan and Perarivalan.
It was their last formal hope of escaping capital punishment.
The condemned men belonged to Sri Lanka's Tamil Tigers militant group and were convicted for plotting the May 1991 killing of Mr Gandhi by a female suicide bomber as he addressed an election rally in the southern Indian town of Sriperumbudur.
In 2006, the Tamil Tigers expressed "regret" for the murder.
The Supreme Court in 1999 confirmed the death sentences of the three men, but commuted the capital punishment to life imprisonment for Nalini Sriharan, an Indian Tamil woman who was also convicted.
Mr Gandhi's mother, former Premier Indira Gandhi, was assassinated in October 1984. One of the surviving killers for that killing was hanged.
The last execution in India was in 2004 when a 41-year-old former security guard was hanged for the rape and murder of a 14-year-old schoolgirl.