Italy on Wednesday told an arbitration tribunal in the Hague on Wednesday that Italian marine Salvatore Girone must be allowed to return to his homeland otherwise he risks four more years in India.
Girone is one of two Italian marines accused by India of killing two Indian fishermen during an anti-piracy mission in 2012.
He has not been able to leave India, aside from a few brief permits, since the incident, Ansa news agency reported.
The other marine, Massimiliano Latorre, is back in Italy after suffering a stroke in 2014.
The Italian government has taken the marines' case to international arbitration after repeated delays in its handling in India.
Ambassador Francesco Azzarello, Italy's representative to the tribunal, said on Wednesday that the arbitration "could last at least three or four years".
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It means that Girone risks "being held in New Delhi, without any charges being made, for a total of seven-eight years".
Azzarello added that this would amount to a "grave violation of his human rights".