He expressed the fear that if Chishty is not released forthwith, he may die in the jail by the time his appeal is decided by the Rajasthan High Court.
In an e-mail sent to the Prime Minister through Member of Parliament Rajiv Shukla, Justice Katju said Singh should grant pardon to Chishty under Article 72 which empowers the President to sanction pardon or remission for convicts.
Justice Katju said he was writing the letter through Shukla as he did not have the Prime Minister's person e-mail ID.
In 2010, Chishty was awarded life sentence under 302 IPC in a murder case by a sessions court in Rajasthan after an 18-year trial. He had filed an appeal in the high court but his bail plea had been rejected.
During the trial period, the sessions court while granting him bail had ordered Chishty not to leave Ajmer. He was re-arrested after his conviction.
In his letter, Justice Katju said, "I am making this appeal to you not as a Supreme Court judge, but as a human being, requesting for release of Dr Khalil Chishty, a Pakistani national, who is old and infirm and (is) in Ajmer jail, on humanitarian grounds under Article 72 of the Constitution."
Chishty, who was an eminent Professor of Virology in Karachi Medical College, holds a PhD from Edinburgh University.