The family of a British man who has been detained in India for seven years without trial has urged the UK Government to intervene.
Christian Michel is accused of bribing Indian officials to win a multimillion-dollar helicopter contract for British-Italian defence company AgustaWestland, which was signed in 2010.
He denies the charge.
The 63-year-old was extradited from the United Arab Emirates to India in 2018 and has been in custody in New Delhi's Tihar Jail ever since.
Christian has served the maximum sentence for the alleged offence he was extradited for - seven years - but has still not been released.
This is despite an Indian law which permits under-trial prisoners to seek release if they have served more than half of the maximum sentence for their alleged offences.
Christian's son Alois Michel, 28, said the UK Government has so far done "nothing" to help his father, but he met with Foreign Office minister Catherine West on Tuesday to urge the Government to press for his father's immediate release.
Alois told the PA news agency: "My father Christian Michel is a British citizen. He has been illegally detained in India and has already completed the maximum sentence of seven years for the charge against him; that, too, without trial.
"Even the UN Working Group on Arbitrary Detention in 2020 confirmed that serious human rights violations were taking place and that he should be released immediately. The UK government, however, did nothing.
"We have requested the UK Government to file an application to the International Court of Justice, given there is no judicial option left for my father in India...
"The British people gave the Labour Party a strong majority in the hope of restoring the honour and strength of our country.
"But so far our hopes haven't been fulfilled. We now truly expect them to act against the unlawful behaviour of a Commonwealth country."
He added that India has "violated its own constitution and international law" by failing to abide by its own Extradition Act, which asserts that no new charges can be added following extradition to India unless the person commits a new offence after their arrival.
Christian was initially accused by India's Central Bureau of Investigation (CBI) of being a key intermediary in the AgustaWestland VVIP helicopter deal, which led to a loss of approximately £344 million to the Indian exchequer, and was accused by the Enforcement Directorate (ED) of receiving around £25.9 million as kickbacks from the deal.
He is now also facing additional charges of forgery of valuable security, which carries severe penalties including life imprisonment.
Alois told PA that this charge was added "in an illegal manner" and that it is being used as a reason not to let his father leave prison.
He added that, although his father was offered bail by the Delhi High Court and the Supreme Court, they are afraid that if he accepts, he will then be charged with "another trumped-up offence".
He said: "In India, today, there is no accountability, no protection and no security."
Alois said he met Foreign Office minister Ms West with "some clear requests from my father", the main one being that the government brings the case to the International Criminal Court (ICJ).
He explained that he did not manage to get any commitments from her, but was told that they will "have a look" at his requests.
Alois said he walked away with "mixed feelings", because: "I am grateful that we are for the first time being heard by government, but I'm not seeing any actions at the same time which makes me even more desperate because they are hearing us but they're not doing anything."
However, he was told that his father is "one of their top priorities in international diplomacy", so he hopes they will "make things move forward in the upcoming weeks or months".
Alois spoke of the "emotional gap" left by his father's absence, which he described as "the most joyous person I know" and "the most open-hearted person I've known in my life".
He said: "In seven years, I only had one time with my father where I was able to speak with him for more than five minutes. Most of the time it's less than 30 seconds and the frequency is quite rare."
Alois' sister Alienor Michel, who has not seen her father since she was 14 years old, told PA: "The most important years of my life; which is growing from a little girl to an adult, was spent without my father. Years where I needed him the most.
"I will never get these moments back. I need him to come back home so that he won't miss the time where I grow as a young adult."
The Foreign Office has been contacted for comment.