Pujith Jayasundara had refused to quit after the Easter Sunday blasts in Sri Lanka.

Sri Lanka's police chief and a top defence official should be tried for "grave crimes against humanity" over their failure to prevent Easter bombings that killed 258, the state prosecutor said Monday.

Dappula de Livera said Inspector-General of Police Pujith Jayasundara and Defence Secretary Hemasiri Fernando failed to heed advance warnings of the April 21 suicide bombings of a local terror group.

"The two officials should be brought before a magistrate for their criminal negligence to prevent the April 21 attacks," de Livera said in a letter to the acting police chief.

"Their negligence amounts to what is known under international law to be grave crimes against humanity."

De Livera ordered acting police chief Chandana Wickramaratne to record statements from both suspects and produce them before a magistrate without delay.

The attorney general said a presidential commission of inquiry had found "major lapses" by Fernando, who has since resigned from his position which was directly under President Maithripala Sirisena.

While Fernando is the most senior defence official to face action, Sirisena suspended police chief Jayasundara after he refused to step down over the handling of the attacks.

Jayasundara and Fernando have testified before a parliamentary inquiry and accused Sirisena of failing to follow established protocols in assessing threats to national security.

Authorities have admitted that warnings sent by India of an impending attack by a local group, the National Thowheeth Jama'ath (NTJ), were ignored. Three churches and three luxury hotels in Colombo were hit by suicide bombers.

Some 45 foreign nationals were among the dead and 500 people were injured in the attacks. The tourist hotspot has been under a state of emergency since.

Sri Lanka's State Intelligence Service (SIS) has also been criticised for failing to act on the Indian warnings, but no-one from the state spy service has been put under investigation.

Sirisena, who is also defence minister and law and order minister, has faced allegations that he too could have prevented the attacks.

The president has objected to a parliamentary investigation into the attacks and ordered police not to co-operate. The hearings, however, have gone ahead.

India had shared information about the targets -- gleaned from a terrorist in custody -- as early as April 4, more than two-and-a-half weeks before the attacks.