The so-called "Chennai Six" had been arrested in October 2013 and sentenced for carrying arms on a commercial US ship.
They were held while working for an anti-piracy security company protecting commercial ships off the coast of Africa in the Indian Ocean and have pleaded their innocence ever since.
"I just feel sheer relief finally we are getting our family back together," said Yvonne MacHugh, the fianc of Billy Irving, one of the six men.
The Madras High Court today ruled that all charges against the six men and 29 others arrested with them be dropped with immediate effect, and the fines they were ordered to pay be refunded.
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The Madurai bench directed that all crew members of MV Seaman Guard Ohio can apply to the Indian authorities to get back their passports after which they will be able to travel home to the UK.
She was among the friends and family members who had been leading a campaign in the UK, including lobbying ministers and Downing Street, to secure the release of the men.
Besides Armstrong and Irving, the other British former soldiers who have been in a Chennai jail for four years include NickDunn, Ray Tindall, Paul Towers, and Nicholas Simpson.
In October 2013, Indian coastguards boarded their vessel and arrested them along with 29 other crew members for taking weapons into India's territorial waters.
But a lower court reinstated the prosecution and they were convicted in January last year and sentenced to five years injail.
Since then there have been a series of appeals, includingBritish Prime Minister Theresa May raising the matter with her Indian counterpart Narendra Modi during her visit to India in November 2016 as well as Britain's High Commissioner in India, Dominic Asquith, visiting the men injailinChennai.
The sailors were aboard the American-owned ship which reportedly offered armed protection to vessels sailing through an area known as "pirates' alley" between the Indian Ocean and the Red Sea.
Customs officials and police found 35 guns, including semi-automatic weapons, and almost 6,000 rounds of ammunition on board the ship, which allegedly did not have permission to be in Indian waters.
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