Ministers have drawn up plans for a form authorising others to block life-saving treatment if the person signing the form becomes incapacitated.
People can appoint someone to instruct a doctor on their behalf
People will be able to grant authority to instruct a doctor under the Mental Capacity Act, coming in next year.
They could do so by ticking "Yes" or "No" boxes and answering a two-line question, a government consultation paper on implementing the act suggests.
Health Secretary Patricia Hewitt said it was not a "step to euthanasia".
"All of that was debated very fully in parliament at the time that that law went through," she told BBC Radio 4's Today programme.
"As I understand it, this is simply designed to enable people to retain the rights that they have under existing law should they be in a position subsequently to find themselves incapacitated."
Matthew O'Gorman, of the campaign group Life, said patients needed to know what treatment could be refused.
"With regard to food and fluids it's extremely important, because the patient has to be aware that if they refuse that medical treatment as defined in the act they will starve and dehydrate themselves to death," he said.
Dr Jacqueline Laing, a law lecturer specialising in medical ethics, said: "What this does is to shift responsibility from the medical profession - which initially had to act in the best medical interests of the patient - to what might often be an ill-informed or an ill-equipped third party."
But a Department of Constitutional Affairs (DCA) statement said "any decisions about treatment will not be taken in isolation" and "must be taken in the best interests of the person concerned".
It added that doctors would be legally protected for continuing treatment while awaiting a court ruling in cases where they felt the representative was not acting in the patient's best interests.
Under the plans outlined in a consultation paper from the DCA, people would tick a "Yes" or "No" box on a form to give their representative the power of attorney over their healthcare.
People would then answer a two-line question asking whether the person being granted power of attorney would be able to give or refuse consent to life sustaining treatment on their behalf.
The law already allows people to appoint their friends or relatives to look after their financial affairs if they become incapacitated.
The form is part of a consultation paper issued on 20 January. The consultation will end on 14 April.
In her foreword to the consultation, Baroness Ashton of Upholland said: "It is important to me that we get this right and that people who choose to make lasting power of attorneys can do so under clear, easy-to-follow processes and simple and effective forms."