Italy has warned India that its relations with the European Union will be harmed if two marines accused of killing a pair of fishermen off the Keralan coast in 2012 are tried under strict anti-piracy legislation.

In a strongly-worded statement, prime minister Enrico Letta's office said a move to prosecute Salvatore Girone and Massimiliano Latorre under the Suppression of Unlawful Acts (SUA) maritime security law was "absolutely disproportionate and incomprehensible".

If the Indian supreme court did not decide to treat the case "in its correct scope" at a hearing on 18 February, the statement added, the result would be "extremely serious".

"It would have negative consequences for [India's] relations with Italy and with the European Union, and equally negative consequences on the global fight against piracy," Letta warned. In a message posted on Twitter he wrote: "Italy and the EU will respond".

Girone and Latorre were part of a military security team protecting an oil tanker from pirates in February 2012 when they alleged shot dead two Indian fishermen. They say they mistook the men for pirates, fired warning shots only, and deny killing them.

The case has raised tensions between Rome and Delhi, with the former harshly critical of the fact that the marines have been held for two years and still have not been formally charged.

Last week India's attorney general said that the pair would be tried under anti-piracy laws- but, in a nod to the outrage expressed by Italy when this appeared a possibility, that the death penalty would not be imposed in the case of a conviction.

The supreme court is due to decide next Tuesday whether to uphold or quash the request, Reuters reported.

On Monday Letta's office did not mince its words on what it said would be an "absolutely unacceptable" decision. "Italy is not a terrorist country," it said in its statement, warning that it "reserves the right" to pursue any form of resistance to the suggestion.

"After two years without charge we do not intend to recede from our aim of bringing Salvatore Girone and Massimiliano Latorre home as soon as possible and to see their dignity and their rights acknowledged," it added.

Antonio Tajani, an Italian vice-president of the European Commission, told the Ansa news agency he would be writing to his colleagues in Brussels to demand "strong action" on the case from the EU.

Italy has asked for the marines to be sent home. They are currently living at the country's embassy in Delhi.