Relatives of six former British soldiers jailed in India are urging the prime minister to help free them.
They plan to deliver a 100,000-strong petition to Downing Street, and want the government to act as a guarantor.
The men had been working on a private US-owned ship providing anti-piracy protection when they were arrested over illegally possessing weapons.
They have been in jail since October and have been refused release despite being granted bail in December.
The US maritime company AdvanFort insists their ship had the correct papers.
The handing-over of the petition, which calls for the men to be released and for the UK government’s backing, coincides with a bail hearing for the group.
Billy Irving, from Connel, Argyll; Nick Dunn, from Ashington, Northumberland; Ray Tindall from Chester; Paul Towers from Pocklington, Yorkshire; John Armstrong from Wigton, Cumbria, and Nicholas Simpson, from Catterick, North Yorkshire, were arrested on 12 October and have been in prison in India since 24 October.
They were working for AdvanFort providing anti-piracy protection in the Indian Ocean when their ship, the MV Seaman Guard Ohio, was detained.
The 35-member crew also includes Indians, Ukrainians and Estonians.
The Indian authorities said they had intercepted the American ship and found weapons and ammunition on board, which had not been properly declared.
AdvanFort said India’s coast guard and police had allowed the ship into the port to refuel and shelter from a storm. It said all weapons had been properly registered.
The men were granted bail in December but, after objections from Indian officials, are yet to be released.
Mr Irving’s girlfriend, Yvonne MacHugh, who started the petition on Change.org, has urged the UK government to intervene.
Ms MacHugh said a “security official in India” had told her that if the UK government were to provide assurances, the men would be freed.
The 25-year-old, who will deliver the petition alongside Mr Irving’s brother Colin, said she had just returned from the country, saying it was “the most emotional trip I have ever had to make”.
‘Difficult and important’
She said: “The men had lost a lot of weight, they looked exhausted and had lost hope of ever being released, their morale is at an all-time low.”
The men have “no idea what is going on with their case”, she said.
“It is in our government’s hands to bring back these men who have fought for us as soldiers and now need the help of their country.”
A Foreign Office spokesman said: “This is a difficult and important case, which the prime minister has raised with Indian ministers, as has the foreign secretary.
“We will continue to do all that we can.”
The men were visited “regularly”, the spokesman said.
He added: “While we are unable to demand the release of British nationals, or interfere in another country’s legal processes, we continue to make very clear our interest in this case, and the importance of ensuring that it is resolved as quickly as possible.”