The government has detailed legislation to create "living wills" for those who become mentally incapacitated.

The bill will introduce a legal framework giving a nominated individual the power of attorney over a patient's affairs.

This would not only include the right to make financial decisions, as is currently the case, but also the right to decide what sort of treatment, if any, a patient should receive.

Under the legislation the Court of Protection will determine what action a "deputy" will be able to take.

Some MPs and peers have expressed dismay at provisions allowing for a patient to make "advance decisions" to cease a medical treatment or even nutrition.

Ministers say the legislation is essential if the "confusing and fragmented" legal status quo is to be addressed.

Constitutional affairs minister Lord Filkin said: "At some point in our lives we are all likely to be affected by a lack of capacity to make decisions, either personally, or because someone close to us is unable to make decisions for themselves.

"No clear and reliable legal framework currently exists to guide carers, families and professionals in decision making.

"As a result, people who lack capacity can be excluded from personal decision making and are vulnerable to abuse; carers can be open to criticism or even prosecution.

"It is also difficult to plan ahead for a time when you might lose capacity.

"The Mental Capacity Bill heralds a cultural shift in the way we treat people who lack capacity. Vulnerable people will no longer be labelled 'mentally incapable' and their views forgotten about.

"Instead they will be placed at the heart of decision making, empowered to make as many decisions for themselves as possible. And where they can't, their best interests will be paramount.

"Carers and professionals will be clearer about their legal position."

It has taken 14 years and numerous stages of consultation for the Bill to reach its current form.

But critics say the legislation could introduce euthanasia through the back door.

They say that an early stage dementia sufferer could make a legally-recognised decision that they would refuse artificial nutrition if they were in a persistent vegetative state.

Anti-euthanasia groups are preparing for a drawn out campaign against the legislation - which will be piloted through parliament by the Department for Constitutional Affairs.