"The charge sought by Indian authorities is unacceptable," Prime Minister Enrico Letta said in a message on his Twitter handle. "Italy and the European Union will react."
His comments came after Indian authorities said the National Investigation Agency (NIA), which is probing the matter, would prosecute marines Massimiliano Latorre and Salvatore Girone under the maritime security law SUA. The SUA carries death penalty.
However, India last week removed the possibility of a death penalty but insisted that the marines would still be prosecuted under the anti-piracy law. Now, they face up to 10 years in jail.
Italy also called on India's Supreme Court to "decide on reasonable charges" in line "with the judgement of January 18," when the same court "ruled out the application of the anti-terrorism law", Italian news agency ANSA reported quoting the statement.
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In New Delhi, the apex court today set February 18 as the next date for hearing arguments from both the sides on the use of the anti-piracy law.
The marines allegedly shot dead two fishermen off the Kerala coast in February 2012, sparking diplomatic tensions between India and Italy.
The marines, deployed on the Italian-flagged oil tanker MT Enrica Lexie, have said they mistook the fishermen for pirates. They are now staying in the Italian Embassy in New Delhi awaiting trial.
Rome wants the marines to be tried in Italy, claiming the incident took place in international waters. However, New Delhi said it has the right to try the Italian personnel as the victims were Indians on board an Indian fishing boat.