The four Veerappan associates had been awarded life term for the landmine blast at Palar in Karnataka in 1993.
But the Karnataka government went to the Supreme Court in appeal which awarded them death sentence in January 2004.
Balamurugan, general secretary of People's Union for Civil Liberties, Tamil Nadu told PTI: "The family members received information from Belgaum prison authorities that the mercy petitions of the four have been rejected".
A spokesperson of the home ministry, facing flak for mishandling Parliament attack convict Afzal Guru's hanging, refused to confirm or deny the reports.
Sources said Prime Minister Manmohan Singh was also "upset" at the way the security establishment handled Guru's execution, particularly the decision to keep his family in the dark about the hanging.
Singh had told home minister Sushil Kumar Shinde that it was one thing to be tough on terror but the delay in informing Guru's family was not how "state craft" was conducted.
The President's Secretariat - that had put out details of mercy petitions - also removed the documents relating to old and new mercy petitions from its website on Wednesday.