A former vet being held on remand in prison on firearms charges is over 20 days into a hunger strike in a protest over a court appearance.
Maurice Kirk, 64, from Llantwit Major, Vale of Glamorgan, has been refused permission to appear in court for a bail hearing rather than by video link.
Mr Kirk, who was struck off the veterinary register, is being held in the hospital wing, say his family.
The Prisons Service confirmed a prisoner in Cardiff was refusing food.
Mr Kirk, who was struck off by the Royal College of Veterinary Surgeons seven years ago for "disgraceful" conduct, is an amateur pilot and a former drinking friend of the late actor Oliver Reed.
He was arrested and threatened with deportation from the United States after landing a plane near President George Bush's Texas ranch last year while on a round-the-world flight attempt.
The case concerns an allegation that he bought a replica World War One bi-plane which came with a decommissioned and triggerless Lewis machine gun.
It is claimed he sold the gun a year ago and was arrested in June by South Wales Police for possession of a firearm and offering it for sale without proper authority.
He was released on unconditional bail by Barry magistrates but the police successfully challenged the decision to free him.
Mr Kirk has said under the law of habeas corpus he has the right to appear in person in court to argue the case for being released on bail.
Mr Kirk's wife Kirstie, 49, said: "I am very worried about my husband - he can only go on like this for another 10 days at the most.
"Maurice is a man of principles and he is also very stubborn.
"He already has health problems and he is losing weight.
"I've asked him to eat but he has no intentions of taking food until he considers he is treated fairly."
She said their 10-year-old daughter was "distraught with worry".
A Prisons Service statement read: "A prisoner at HMP Cardiff is refusing food.
"Guidance to prison staff on managing food refusal states that food refusal should always be taken seriously.
"The prisoner should be helped to find constructive ways to meet his or her underlying need, for example through the application process, the Independent Monitoring Board, Request and Complaint procedure or contacting a lawyer or MP.
"A multi-disciplinary approach to managing those refusing food is encouraged, and healthcare staff should be involved at an early stage in order to assess the prisoner's physical and mental health.
"Healthcare staff may wish to involve their local primary care trust for specialist dietary advice."
Mr Kirk's MP John Smith has taken up the case and is awaiting a response from the ministry.
Mr Kirk's legal representative Patrick Cullinane added: "We are really worried that they will be bringing him out of that prison in a coffin.
"I have known Maurice for a long time - he is a strong-willed man so I don't expect him to back down."