Ahead of Monday's ruling in the Supreme Court on a petition by the Italian government to rule out use of a harsh anti-terrorism law in the case, De Mistura said on Monday will be "not the day of judgement, but of truth". (Agencies)
He set out three possible scenarios.
The first "is that the attorney general says that the police can press charges under the Sua Act (anti-terror Suppression of Unlawful Act). In this case we will oppose the decision with all our strength," De Mistura told an Italian news agency.
The second "is that the prosecution proposes a different formula under the Sua Act, but which excludes the death penalty, in the hope of relieving international pressure," added De Mistura.
"This too would be unacceptable because Italy contests the use of the anti-terrorism law," he said.
The third possibility is that India further postpones a decision.
In this case, De Mistura said Italy would contest the move on grounds that official charges have still not been issued after two years and that it is time to bring the marines home.
The two Italian marines, Massimiliano Latorre and Salvatore Girone, are facing trial in India for allegedly killing two Indian fishermen after allegedly mistaking them for pirates and opening fire on their trawler.
Latorre and Girone were guarding the privately owned Italian-flagged oil-tanker MT Enrica Lexie off the coast of Kerala on February 15, 2012.
On Thursday, speaking to Italian journalists, the two marines said they were sorry for the deaths but denied all responsibility pending a decision by the Supreme Court on use of a severe anti-terrorism law in their prosecution, said the news agency.
"We are sorry about the loss of human lives, but in no way do we feel responsible," said Girone.
"We feel sorrow on a human level, but we are innocent," he told journalists in New Delhi.
The pair said their sorrow was compounded by the fact that the fishermen were seamen like themselves.
"We both grew up in two cities on the sea and we are fishermen ourselves," they said. "Like them, we are seamen".
Italian Premier Enrico Letta said on Thursday that bringing back the two marines is the government's top priority.
"From my window (in the premier's office) I can read the banner on the facade of the building that houses (newspaper) Il Tempo: 'Let's bring the marines home'," the leader of Italy's left-right coalition government wrote in a letter to the daily, published on Thursday.
The Italian government has petitioned the apex court to rule out use of the Sua law and for the servicemen to be allowed to return to Italy.
A ruling is expected on February 10. Rome argues that prosecuting them under the Sua Act de facto equates the incident with a terrorist act - an accusation which Latorre said "pains us not just as servicemen, but also as parents and as human beings," said Latorre.
"As a professional Italian serviceman who fights piracy, I am very upset by this," Latorre added.
Italian Foreign Minister Emma Bonino on Thursday said the marines "are neither terrorists nor pirates".
Ahead of Monday's ruling in the Supreme Court on a petition by the Italian government to rule out use of a harsh anti-terrorism law in the case, De Mistura said on Monday will be "not the day of judgement, but of truth".