Ian Lavery, a Labour MP from Northumberland in northeast England, has been campaigning for his constituent Nick Dunn as well as Billy Irving, Ray Tindall, Paul Towers, John Armstrong, and Nicholas Simpson.
The former soldiers who worked on an anti-piracy ship were arrested on firearms charges in October 2013 when the US-owned ship they were on was found to be full of weapons.
"This verdict will have come as a hammer blow to Nick Dunn and the other men detained in India coming only a matter of months after a court had quashed all of the charges," Lavery told The Yorkshire Post.
"The men and their families will be distraught at this news with all noises coming out of the courts in the last few days and weeks pointing towards them returning home in the near future after years of hell."
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"I will continue to work with Nick's family to fight for his release and return to the UK and (I) am seeking urgent discussions on the way forward and seeking an early resolution to this miscarriage of justice."
All men aboard the ship - which include the six Britons, 12 Indians, three Ukranians and 14 Estonians - received five-year sentences and were ordered to pay Rs 3,000 by Tutucorin district sessionscourt.
The UK Foreign and Commonwealth Office said the men had 90 days to appeal against the decision.
"Our staff in India and the UK have been in close contact with all six men since their arrest to provide support to them and their families, including attending court," a spokesperson said.
The MV Seaman Guard Ohio, which flew the Sierra Leone flag, was found about 15 nautical miles from Tuticorin port in October 2013.
It is owned by Advanfort, a US company that specialises in maritime security and offered armed protection services to vessels sailing through an area known as "pirates' alley" between the Indian Ocean and the Red Sea.