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.”

Mr 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, Mr Singh’s rivals in the main Opposition the 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.”