Upholding a 2003 high court verdict awarding the death sentence to Mohammed Afzal, the court ruled that there was not even a shred of doubt about his complicity in hatching the plot to commit the "most diabolical attack on parliament", the Press Trust of India news agency said.

The court commuted the death sentence of another accused, Shaukat Afsan Guru, to 10 years in prison while upholding the high court acquittal of two others, Shaukat's wife Afsan Guru and a professor of Arabic studies in a Delhi college, S.A.R. Geelani.

"All evidence unerringly points to Afzal who was a key conspirator and played an active role," Justice P.V. Reddi and Justice P.P. Naolekar said in a 271-page judgement.

"He is definitely involved in the conspiracy to attack parliament with the use of explosive substances," they said, observing the attack had no parallel in the history of Indian democracy.

Executions, a rare occurrence in India, are conducted by hanging. Afzal can now only appeal to the president for clemency.

Five gunmen stormed into the sprawling parliament complex on December 13, 2001 but before they could enter the building and session houses they were killed by the security forces.