The Mental Health Act Commission has for the first time prepared guidance for health authorities and social services because of confusion over how far doctors can go to stop patients from starving to death.
The legal position has not been clear and the commission took the decision to issue the briefing note after the case of Nikki Hughes, who died in January 1996. Ms Hughes had suffered from anorexia since her teens.
The Royal Shrewsbury Hospital, where she was treated, sought legal advice and was told that feeding her without her consent could have led to assault charges. Doctors were told they could not override her wishes not to eat.
In 1992, however, the High Court ruled that a 16-year-old orphan, known as "J" should not be allowed to starve herself to death, but could be treated against her will to prevent her suffering serious brain damage.
The commission's briefing note advises doctors that in certain situations, patients with anorexia nervosa, whose health is seriously threatened, may be compulsorily admitted to hospital.
While "the consent of the patient should always be sought", it goes on to say that some patients "may not be able to make an informed choice as their capacity to consent may be compromised by fears of obesity or denial of the consequences of their actions".
It adds: "The courts have ruled that feeding a patient by artificial means to treat the physical complications of anorexia nervosa can reasonably regarded as medical treatment for a mental disorder."
The move was welcomed by mental health experts and by pressure groups representing sufferers. A spokeswoman for the Eating Disorders Association said: "Any guidelines which help people interpret the Act surely must be good to clarify the situation for people."
Dianne Jade, principal and founder of the National Centre for Eating Disorders, said: "I have always approved of compulsory treatment for anorexics. In the condition the thinking processes in the brain are changed by starving and the patient is not in control or able to take care of herself."
Dr Jill Welbourne, an eating disorder specialist, said that it was a "step forward" for treatment of anorexics. "It explains that anorexia is a mental illness, health and safety is at risk and that voluntary treatment is not always possible or practicable."
June McKerrow, director of the Mental Health Foundation, said: "It makes the position clearer for clinicians and carers." But she called for anorexics to have access to treatment much earlier in their treatment: "Anorexia is a growing problem and unless ways of treating people are sorted out we will see more and more people reaching this unnecessary late stages of crisis."
As many as one in 20 people, of which the vast majority are women, display symptoms of anorexia, although most are never formally diagnosed. About one sufferer in 100 needs long-term treatment, and of these one-fifth die, half from starvation and half from suicide.