ROME—Italy has decided not to send two marines back to India, where they stand accused of fatally shooting two local fisherman, the Foreign Ministry in Rome said Monday, a move likely to deepen a diplomatic rift between the two countries.

"Italy has always held that Indian authorities violated international law," the ministry said. It added that "Italy has informed the Indian government that, since an international dispute between the two countries has now been formalized, the marines Massimiliano Latorre and Salvatore Girone will not be returning to India at the end of their leave."

Messrs. Latorre and Girone are accused of killing two fishermen while on board a commercial Italian ship off the coast of the Indian state of Kerala last year, mistaking them for pirates. The men deny that they committed murder.

India's Supreme Court in February let the two men return to Italy to vote in general elections and return after four weeks, a term that just expired. At the time, Italy vowed the two marines would return to India.

The marines, who were being held at the Italian Embassy in India and reported to a local police station once a week, were also allowed to return to their families for Christmas.