India's Prime Minister Manmohan Singh warned Italy on Wednesday that there would be "consequences" unless it returned two of its marines who skipped bail while on trial in New Delhi for murder.

Reflecting growing anger in India, the normally mild-mannered Singh told MPs it was "unacceptable" that both men had remained in Italy after being allowed home to vote in elections last month.

"This cannot by any standards be in the interests of any bilateral relationship that has to function on the basis of trust," said the premier, whose government is under intense pressure to take action against Italy.

"Our government has therefore insisted that the Italian authorities respect the undertakings they have given to the honourable supreme court and return the two accused persons to stand trial in India.

"If they do not keep their word, there will be consequences for our relations with Italy."

Singh's comments come after Italy's ambassador, Daniele Mancini, was summoned to the foreign ministry on Tuesday, where India's Foreign Secretary Ranjan Mathai demanded that Rome immediately hand over the marines.

Reports in newspapers including the Times of India and the Indian Express have said that Mancini could now be expelled.

Massimiliano Latorre and Salvatore Girone are accused of shooting dead two fishermen off India's southwestern coast in February last year, when their fishing boat sailed close to the Italian oil tanker they were guarding.

The marines said that they mistook the fishermen near the port of Kochi for pirates. They were granted leave by India's top court to return home for four weeks in order to vote in the February 24-25 national elections.

But the Italian foreign ministry announced on Monday that the pair would not return to India once their leave expires in view of a "formal international controversy" between the two countries.

Italy insists the marines should be prosecuted in their home country because the shootings involved an Italian-flagged vessel in international waters, but India says the killings took place in waters under its jurisdiction.

The case was initially heard in a local court in India's southern state of Kerala but it was later transferred to the Supreme Court in New Delhi which ordered that a special court be set up for the trial.

Relations between the two countries have also been soured by corruption allegations surrounding a $748 million deal for the purchase of 12 Italian helicopters which the Indian government is now threatening to scrap.

In the wake of the announcement that the marines would remain at home, Singh's rivals in the main opposition Bharatiya Janata Party (BJP) accused Italy of treating India like a "banana republic".

The Times of India reported on Wednesday that the government was considering expelling Mancini as he had given his personal assurance before the supreme court that the two marines would be return.

Even the marines' Indian lawyer, Harish Salve, has washed his hands of the pair.

"It was an act of faith in a friendly government that the government of India did not oppose this request (for leave), and an act of grace on the part of the Supreme Court of India to grant the permission sought for," he said in a statement on Wednesday.

"I consider this action of the Republic of Italy as a breach of faith. It is my perception that the Italian government should have, in the least, forewarned its Indian lawyers of the change of its position before communicating it to the government of India.

"In these circumstances, I have informed the Italian ambassador that it will no longer be possible for me to appear, for me to be associated with this case."

India and Italy continue row over trial
New Delhi (UPI) Mar 13, 2013 - A lengthy diplomatic row between New Delhi and Rome deepened after Italian police refused to return two Italian marines to India to stand trial.

Indian Prime Minister Manmohan Singh said Italy's refusal to send the two Italian naval guards back to India to face trial for killing two fishermen was "unacceptable," a report by The Times of India said.

The two guards are facing trial in a special court in New Delhi on charges they killed two fishermen off the Kerala state coast in February 2012.

The marines were guarding an Italian oil tanker off India's southwestern coast when the shooting happened. The fishermen were allegedly thought to be pirates, a Press Trust of India report said at the time.

India says the fishermen, Selestian Valentine and Ajesh Pinky, were unarmed.

Italy maintains the Indian fishing boat behaved aggressively and ignored warning shots from the guards on the oil tanker Enrica Lexie.

A diplomatic row quickly ensued after the arrest of the guards by Indian maritime authorities, who took the marines to the city of Kollam in Kerala.

Rome says the incident happened in international waters where the Indians don't have jurisdiction and that Italian nationals must be tried in Italy. However, in January India's Supreme Court rejected the Italian government's case for transferring the marines who had been out on bail since June.

The Kerala High Court granted conditional freedom to Latorre Massimiliano and Salvatore Girone on a surety of around $200,000 each and on condition they surrender their passports, The Times of India reported.

The marines also were under strict reporting orders and had to present themselves to the police daily, as well as remain within 6 miles of the main police station in Kochi -- formerly Cochin -- in Kerala.

The marines' trip to Italy was their second since their arrest in February 2012. India's Supreme Court allowed the men to go home for Christmas for two weeks, a report by the BBC said.

A civil case over the shooting deaths was settled in late April when each family of the dead fishermen accepted around $190,000 as compensation and for dropping the charges, a Press Trust of India report said.

But Indian authorities didn't drop the criminal case, for which the marines remain charged.

Italy has complained in the past of the long time Indian authorities are taking to move the case to trial.

India's External Affairs Minister Salman Khurshid said he will take up the latest developments with Italy, The Times of India reported.

"We will study and take a rightful, informed position," he said.