Nick Dunn could find out on Monday whether he will be freed on bail while the appeal against his conviction is lodged.
Nick, from Ashington, Northumberland , is one of six former British servicemen currently serving six-year prison sentences in India for weapons offences.
A court found they had entered Indian waters in 2013 carrying unlicensed firearms but the men say they were defending merchant ships from pirates and deny all charges.
An Indian judge will hear the men’s case on Monday and could make a decision on their bail application as soon as Monday night. Lawyers for the former soldiers can move on with the appeal against their convictions.
Nick’s sister Lisa has campaigned tirelessly for her brother’s release, She said: “Getting him home for Christmas would be the best present in the world, but that is not looking likely unless we get a miracle. Though you can fly on Christmas Eve, so you never know.
“If the lads get bail that would be a huge relief. At least then he would no longer be under those horrendous conditions and could better look after himself.”
Last month Theresa May flew to India to discuss the men’s plight with Indian Prime Minister Narendra Modi.
During almost three hours of talks in New Delhi with Mr Modi, Mrs May told the Indian PM that the plight of the so-called Chennai Six was “a case of concern” in the UK.
British sources said that Mr Modi assured Mrs May he was aware of UK concerns and held out the prospect of considering whether there was anything that could be done by the two governments once the current appeal process has concluded.
The British Government has not taken any position on the allegations against the men and has made clear it respects the independence of the Indian judiciary.
Ministers have already raised the case around 40 times – including five times with Mr Modi – but diplomatic action has focused on ensuring the matter is resolved as swiftly as possible.
Alongside Nick, the imprisoned ex-servicemen are John Armstrong, 28, from Cumbria; Ray Tindall, 41, of Chester; Nicholas Simpson, 45, of Catterick, North Yorkshire; Paul Towers, 52, of Pocklington, East Yorkshire; and Billy Irving, 35, of Oban, Scotland.