Number 10 has told ITV News Tyne Tees it has raised the issue of Nick Dunn's arrest and imprisonment with the Indian government 45 times - and 5 times with prime minister Narendra Modi.
A spokesperson for Theresa May has said she will raise the issue with him again later on a trade mission to the country.
Nick Dunn was one of three decorated north east soldiers aboard the armed security ship MV Seaman Guard Ohio in 2012 when the crew was arrested in Indian waters.
He, Nicholas Simpson from Catterick and Paul Towers from Pocklington near York, all insisted they were working legitimately providing properly authorised protection from pirate attacks to other ships.
But a court found them guilty of weapons offenses and they were sentenced to five years in jail in January.