MPs are to debate a bill which would give people the legal right to draw up so-called "living wills".
Some 2m Britons are unable to make decisions for themselves
What are living wills?
A living will is a document that spells out how an individual want to be cared for if they become seriously ill, for instance, and are no longer able to be make their own decisions.
They enable people to make advance decisions on how they should be treated.
People can use the wills to state that they do not want treatment to prolong their life if they are dying.
For instance, they can refuse in advance to be resuscitated or to be fed through a tube.
But critics fear patients could make uninformed decisions, which may lead to doctors withdrawing food and fluids to comply with living wills, even if they think it is inappropriate.
What are the MPs discussing?
The proposals concerning living wills are contained in the Mental Capacity Bill, which has reached the report stage in the parliamentary process.
MPs are discussing a proposed amendment to the bill, aimed at preventing decisions being taken that would bring about death which has been tabled by former Conservative leader Iain Duncan Smith.
Ninety one MPs have signed a petition backing the amendment.
The Bill still has to go through a third reading and be debated in the Lords before it can become law.
Why are critics concerned?
There are fears that people may not fully understand their diagnosis when they make a decision that they would not want to receive medical treatment if they became severely incapacitated.
It is also feared some people may change their mind after they have signed a living will - or when they do become ill and are no longer able to communicate their wishes.
The Christian Medical Fellowship, which opposes the current proposals, said it believed advance refusals of treatment should be only advisory, not legally binding.
Will this make euthanasia legal?
No. The government is adamant that the Bill will not affect the law on murder, manslaughter or assisted suicide.
As a result, euthanasia or asking somebody to help you die remains illegal.
The government has emphasised that it is against euthanasia, but it says doctors and patients want more clarity in relation to living wills.
Can living wills be used for anything else?
Living wills can be used to name an individual to take decisions for you, if you no longer have the ability to do so yourself.
At the moment, people can appoint an "advocate" to look after their financial affairs if they become incapacitated.
However, they have no legal right to appoint someone to make decisions about their health and welfare, which would enable them to make decisions on whether or not the patient should receive life-prolonging treatments.
Naming somebody in a living will can enable people to get around this problem.
However, the government is planning to change the law to ensure people can appoint advocates who can also take decisions on their health and welfare.