The families of six British men jailed for five years on firearms offences in India this year are calling on the Prime Minister to intervene.
The UK's Foreign Office has told the BBC that PM Theresa May will petition Indian Prime Minister Narendra Modi when she visits India early next week on a trade mission.
But the sister of one of the men, Nick Dunn, has told the BBC's Victoria Derbyshire programme that just raising the issue is "not good enough."
She said May "cannot waste this opportunity face-to-face with Modi, she has six British men at her mercy, and can't just waste it talking about her trade deals."
May is making the trip to establish post-Brexit ties with India and talk about the future of trade between the two Commonwealth nations. India's economy has grown rapidly in the past 15 years and is the world's second largest populace with a market of 1.2 billion people.
"Theresa May has the power the end this now, and that's what we want her to do," said Dunn, adding it would be "an absolute travesty if she fails to raise" the plight of the men.
All former British soldiers, the six men were arrested in October 2013 after the private anti-piracy vessel they were working on crossed into Indian waters without permission. It was patrolling an area between the Indian Ocean and Red Sea notorious for piracy.
Onboard, authorities found 35 weapons and ammunition. Dunn and 24 other crew on the ship were arrested for being in Indian waters without documentation and with unauthorized weapons. The ship, the MV Seaman Ohio, was operated by the US-based anti-piracy security firm AdvanFort.
The charges against Nick Dunn were initially dismissed and he was released, but police didn't hand back his passport or those of the other men. They were then put on trial and sentenced to five years in prison in January 2016.
Last year former Prime Minister David Cameron also petitioned Modi for the mens' release before the Indian Prime Minister visited the UK in November. But the Indian court pressed ahead with the case.
Dunn's sister Lisa told the BBC that she is "sick to death" of the UK government telling her that it's unable to interfere in another county's legal system.