A former British serviceman who spent four years in an Indian prison has said he was let down by the UK government and that Boris Johnson did “nothing” as foreign secretary to secure their release.

Nick Dunn, 34, was one of six British men who were detained while working as security guards on the MV Seaman Guard Ohio, an anti-piracy vessel, in the Indian Ocean in 2013.

The former soldiers were charged with carrying unlicensed firearms and ammunition and held in often squalid prison cells in Chennai for four years until they were acquitted in November 2017.

The men insisted they had been granted permits by the UK’s Department for Business, Innovation and Skills, something the head of export controls eventually confirmed to the Indian authorities.

The case of the so-called Chennai Six was raised at the highest level by David Cameron as prime minister to his Indian counterpart, Narendra Modi, but it was only after years of campaigning by their family and friends that they were acquitted after appealing against their convictions.

Speaking in detail for the first time about his experience, Dunn said he respected the efforts of Foreign Office staff on the ground in India but felt that senior politicians – including Johnson – did not do enough to secure their release.

“I feel let down. I don’t expect to have special treatment just because I served my Queen and country,” said Dunn, who served with the British army in Iraq, Afghanistan and Northern Ireland.

“I have my rights as a British citizen. If things went down in a foreign country, you should be there to assist me. They knew we were innocent, so treat us as innocent. I think the British government could have done more.”

Dunn, from Ashington in Northumberland, said he lost 10kg (one and a half stone) after he and his five compatriots became ill in the cramped prison cells in Chennai.

The former Parachute Regiment soldier said he felt the UK government failed to understand the nature of the case or grasp the men’s innocence, and that ministers were reluctant to start a diplomatic row with India as it might jeopardise a future trade deal.

In a book released on Thursday, called Surviving Hell, documenting his time in prison, Dunn said the case was passed from the former foreign secretary Philip Hammond – whom he described as a “complete waste of space” – to his successor Johnson.

He said Johnson promised to “leave no stone unturned” to free the men. “I think it would be fair to say that he left a lot of stones unturned. What did he do for us? Nothing, as far as I could tell. He was all mouth.”

He added: “Since Boris Johnson has become prime minister, a job he had always wanted, I have never believed a single word he said about anything.”

The six Britons – Billy Irving, from Paisley in Renfrewshire, Ray Tindall from Chester, Paul Towers from Pocklington, east Yorkshire, John Armstrong from Wigton, Cumbria, and Nicholas Simpson, from Catterick, North Yorkshire – were arrested and detained along with three Ukrainians, 14 Estonians and 12 Indians when customs officials and police searched their ship.

Dunn said the UK government only intervened in cases involving its citizens abroad when they had been convicted. By then it was often too late. He said the Foreign Office should intervene much earlier when there is clear evidence to prove the innocence of those charged with criminal acts abroad.

He said: “Our government should put the interests of its own citizens before trade deals. No one should ever have to feel that people at the highest level are more worried about offending a foreign government than letting their citizens rot.

“I had served my country as a soldier. I put my life on the line as a paratrooper in two war zones and I had almost died in the line of duty for my Queen and country – but where was my country now? I have always been proud to be a member of an elite regiment of the British army but when I really needed the government on my side, it failed me.”