Mr Johal, a Sikh from Dumbarton, remains in detention despite widespread calls for his release, including a call earlier this year from the United Nations working group on arbitrary detention, which said there is “no legal basis” for his imprisonment.

He was arrested shortly after travelling to the Punjab for his wedding in November 2017. He denies being involved in a plot to assassinate Hindus.

The UN report states that Mr Johal was “subjected to torture” and that he was “targeted because of his activities as a Sikh practitioner and supporter and because of his activism in writing public posts calling for accountability for alleged actions committed against Sikhs by the authorities”.

A legal claim filed by Mr Johal’s legal team at Leigh Day, supported by NGOs Reprieve and REDRESS, alleges that British intelligence agencies MI5 and MI6 provided the tip-off to Indian authorities that led to his arrest.

His lawyers argue that he appears to be the individual described anonymously in a report by the UK’s Investigatory Powers Commission’s Office (IPCO), which sets out how MI5 and MI6 passed information about a British national to foreign authorities who then detained and tortured them.

Mr Johal is asking the UK government to grant him redress for the harm he has suffered and recognise its actions were unlawful. He has also sought a public apology for the government’s role in his suffering.

Gurpreet Singh Johal, Mr Johal’s brother and a practising solicitor in Scotland, said: “I never imagined the scenes I’d seen in horror movies of people being abducted by a foreign government and violently tortured could become a reality for someone in my family.

“When it did, I expected our government would do everything in its power to save my brother. The twist is that not only has our government abandoned one of its own citizens, it’s actively betrayed him.”

He added: “If this can happen to my brother, it could happen to any British national traveling overseas. Jagtar’s only ‘crime’ was writing blogs exposing the Indian government’s mistreatment of Sikhs, and the United Nations says this is the reason he’s been targeted.

“The UK should be championing free speech around the world, not assisting repressive regimes to torture and lock up British nationals who dare to criticise them. We need answers and accountability to make sure no other British family is put through this living hell.”

Leigh Day partner Waleed Sheikh said: “Our client is a British man from Dumbarton who has now been detained in an Indian prison for more than four and a half years, without prospect of trial or release in the near future.

“It is vitally important for our client and his family to understand whether or not and to what extent the UK authorities were involved in sharing intelligence that may have led to our client’s arrest and subsequent detention and torture.

“It would be totally unacceptable for the UK government’s actions to have placed an individual, let alone a British citizen, at risk of torture or the death penalty.”

Rupert Skilbeck, director of REDRESS, said: “Jagtar’s case demonstrates how important it is that there is a full review of the way that the UK government responds when UK citizens are tortured abroad, including a careful examination of the role of the intelligence community in this case.

“Where there is any suggestion of collusion then there must be a full investigation, as there can be no immunity for torture.”

Maya Foa, director of Reprieve, said: “It was already a scandal that when a British national was snatched off the street by Indian authorities while on his honeymoon, Boris Johnson left him to rot for five years before finally admitting he was being arbitrarily detained.

“Now it appears the UK government hasn’t just been negligent, but may have unlawfully enabled his abduction and brutal torture through a tip off to the Indian authorities.

“The very least we can expect of our government is not to share intelligence that leads to us being detained and tortured overseas. We’re talking about a British blogger who United Nations experts say has been targeted because he spoke out against abuses committed against the Sikh community in India – the UK government must finally bring him home.”