A dispute between India and Italy over the case of two missing Italian marines accused of killing Indian fishermen has taken another dramatic turn, with India’s supreme court rejecting the Italian ambassador’s right to diplomatic immunity and ordering him not to leave the country.

Chief Justice Altamas Kabir appeared incensed by the Italian envoy Daniele Mancini’s failure to honour an undertaking given to the court earlier this year that the two marines, Massimiliano Latorre and Salvatore Girone, would return to Delhi to stand trial.

The marines, who were stationed on an Italian oil tanker, were detained by the Indian authorities in February last year after they shot and killed two Indian fishermen off the Kerala coast, having apparently mistaken them for pirates.

The Indian court order prohibiting Mancini from leaving the country came four days after Rome issued a note verbale saying its envoy’s movements could not be curbed under the 1961 Vienna convention governing diplomatic relations between sovereign nations.

But Kabir said: “We have lost our trust in you … You have given a personal undertaking [to the court],” he told Mancini. “Those who appear as petitioners and give an undertaking cannot claim immunity.

“Some people are writing we are naive. But we didn’t expect the republic of Italy to behave like this. What do they think of our courts … our judicial system?” asked the judge.

Even as the court’s order triggered a legal debate over a diplomat’s legal rights and privileges under the Vienna convention, Delhi could be faced with a difficult situation if Mancini were to try to leave India.

The foreign ministry spokesperson Syed Akbaruddin told journalists on Monday that while Delhi was aware of the provisions of the convention, it was also bound by the supreme court’s order.

But what if Mancini were to attempt to visit Nepal, a neighbouring country to which the Italian envoy is also accredited, Akbaruddin was asked. He sidestepped the question by saying the foreign ministry had no such information on Mancini’s plans.

India’s relations with Italy have come under severe strain in recent months not just because of the killing of the two fishermen but also because of a bribery scandal involving the purchase by India of Italian-made AugustaWestland helicopters. Delhi also has to worry about the fact that the ruling Congress party is led by the Italy-born Sonia Gandhi, a fact that opposition politicians are quick to exploit.

But the bigger concern in Delhi is that the two missing marines could seriously damage relations with Italy – and even create problems with the European Union. The prime minister, Manmohan Singh, has already warned that if the Italians “do not keep their word, there will be consequences”. However, with public opinion in Italy also aroused over the issue, there remains a big question mark over the return of the two marines to Delhi.

2013 Guardian News and Media Limited