Dr Percy Brown, senior medical officer at Belmarsh prison, south-east London, said Jesus Marquez-Neira, 37, a convicted Colombian drug smuggler, was completely sane and fully understood that his persistent refusal to take nourishment would lead to his deterioration 'to the point of death'.
The doctor told Mr Justice Macpherson that there had been a gradual deterioration in Neira's condition over the past three to four weeks. His weight was down to about 41kg (90lbs) and he could walk only with difficulty.
Although Neira had taken some food since the start of this week's court hearing, 'to alleviate stomach pain', he was insisting that he would continue his fast if kept in prison.
A father of four, he went on hunger strike in mid-May after protesting his innocence and stating that he would rather die than serve an 11-year sentence imposed at Croydon Crown Court in February for smuggling cocaine worth pounds 250,000 into Britain.
Kenneth Clarke, the Home Secretary, is seeking court declarations that the authorities, physicians and medical staff at Belmarsh prison can lawfully 'observe and abide by' Neira's refusal to take food, refraining from providing nutrition until he consents to receive it.
Mr Clarke is also seeking permission from the courts not to have to provide Neira, until he consents, with medical treatment intended to prolong life if his condition deteriorates as a result of his hunger strike.
The case is the first test of a change in law on forcible feeding in 1974. Up until then anyone - even if mentally capable of expressing a wish to starve - would have been kept alive. After 1974 the law obliged the Home Office to abide by the wishes of anyone deemed mentally capable of making a reasoned choice. Although there have been other prolonged hunger strikers since the new legislation, they have ever reached the critical stage of Neira.
Mr Justice Macpherson said that, in ordinary terms, it would be 'a gross assault and invasion of his body' to force feed or treat Neira without his permission, unless it could be shown that he lacked the mental capacity to refuse nourishment.
Dr Brown said: 'That is precisely why I need the protection of the court.'
The doctor said he was in daily contact with Neira in the prison hospital.
Nicola Davies QC, for the Home Secretary, told the court: 'It is our case that the interest of the patient, and in particular his right of self-determination, is paramount - paramount over the interests of society in upholding the concept that human life is sacred and should be preserved if at all possible.'
The Prison Act placed a duty on those in the prison service to recognise a prisoner's rights and that, implicitly, meant the right to self-determination possessed by every person of sound mind and adult years.
Miss Davies said the latest information was that Neira had started eating this week.
The hearing was adjourned until today.