Marine Salvatore Girone has been living in Italy's embassy in New Delhi for several years and is banned from leaving the city pending resolution of a dispute which has soured bilateral ties.
Girone and fellow marine Massimiliano Latorre were arrested by Indian police in 2012, days after they shot at an Indian fishing boat, killing two fishermen, while protecting an Italian oil tanker as part of an anti-piracy mission off India's southern Kerala coast.
The so-called “Enrica Lexie incident”, named after the oil tanker, is now subject to international arbitration in a bitter fight between the two countries.
Ties soured even further after the two marines overstayed a return trip to Italy they were allowed to make in early 2013 to vote in general elections.
Both marines were barred from leaving India again pending a trial, although they have not yet been charged.
But Latorre was allowed to travel back to Italy in 2014 for medical treatment after he suffered a stroke.
Girone however “is obliged to live thousands of kilometres away from his country and family, with two children still at a tender age” and is deprived of “his liberty,” Italian ambassador Francesco Azzarello told the Permanent Court of Arbitration (PCA).
“He has not been subjected to any charge… his rights are seriously suffering,” said Azzarello at the Hague-based arbitration body, set up in 1899 to rule in disputes between states and private entities.
Girone should be allowed to come home “pending the final determination of this tribunal,” Azzarelo insisted.
The Indian courts in January extended Latorre's permission to stay in Italy until April 30th.
Italy initiated arbitration proceedings last year and, in August a UN body, the International Tribunal for the Law of the Sea (ITLOS), ordered India to suspend court proceedings against the pair.
Italy insists the Enrica Lexie was in international waters at the time of the incident.
India argues the case is not a maritime dispute but “a double murder at sea”, in which one fisherman was shot in the head and the other in the stomach.
The dragging case has become a political hot-button issue in Italy with Prime Minister Matteo Renzi regularly flayed by opposition leaders for failing to secure the release of both men.
Azzarello said on Thursday that Italy “gives the solemn undertaking” the marines will be returned to India if the PCA orders Rome to do so.
However “a human being cannot be used as a guarantee for the conduct of a state,” Azzarello said.