"Under Section 62 (5) of the Representation of the People Act, 1951, a person confined in jail cannot vote," an Election Commission official told IANS.
"Sharmila earlier submitted an application expressing her desire to cast her vote. But we cannot entertain her prayer as per law," he said.
The 42-year-old Sharmila told reporters in Imphal recently: "I never voted as I had lost faith in democracy, but the rise of the new anti-corruption party, Aam Aadmi Party, changed my thinking."
Balloting took place Thursday for the second Lok Sabha seat from the state - Inner Manipur - while polling for the tribal reserved Outer Manipur seat was held April 9.
Sharmila has been on an indefinite fast since Nov 4, 2000, demanding repeal of the Armed Forces (Special Powers) Act, 1958, (AFSPA).
She was charged with attempting to commit suicide. Under this charge, she can be kept in custody for one year at a stretch.
In view of her ill health, Sharmila is currently lodged in a special ward of the Jawaharlal Nehru Institute of Medical Sciences in Imphal where one room, where she is confined to, has been declared sub-jail.
The AFSPA, against which Sharmila has been fighting, provides unlimited powers to the security forces to shoot at sight, arrest anybody without a warrant or carry out searches without hindrances. It also insulates the security forces from legal processes for any action undertaken under the act.