The government today unveiled plans to let people appoint a relative or friend to instruct a doctor to deny them life-saving treatment.

The Mental Capacity Act, which comes into force next year, will allow people to appoint someone to instruct a doctor about what treatment they want if they become too ill to do so themselves.

But ministers denied the proposals amounted to backdoor euthanasia.

Under proposals published in a Department of Constitutional Affairs consultation paper today, people would tick a yes or no box on a form to give a representative the power of attorney over their healthcare. They would then answer a two-line question asking whether this representative would be able to give or refuse consent for life-saving treatment.

People over 18 already have the power to give a friend or relative power of attorney to deal with their financial matters in case they become incapable of doing so themselves.

The health secretary, Patricia Hewitt, today denied the new proposal was a step towards legalisation of euthanasia.

She told the BBC Radio 4 Today programme: "As I understand it, this is simply designed to allow people to retain the rights that they have under existing law should they be in a position subsequently to find themselves incapacitated."

The proposed tick-box form would allow people to assign "lasting power of attorney" to up to four other people, who could be friends, relatives or professionals, such as solicitors.

There would be two separate forms, one empowering others to make decisions over the person's "property and affairs" and the other over their "personal welfare". The personal welfare form would include the tick-box question: "I wish to give my attorney(s) authority to give or refuse life-sustaining treatment on my behalf."

That would be followed by a section where applicants could specify "restrictions and conditions" on their attorney or attorneys. It could be specified that they act jointly.