A WOMAN paralysed from the neck down has died peacefully in her sleep after winning the legal right to have medical treatment withdrawn, reigniting the debate over voluntary euthanasia.

Critics say the law is inconsistent, allowing doctors to switch off life support machines but not allowing them to help patients take their own lives.

Dame Elizabeth Butler-Sloss, President of the High Court family division, ruled last month that Miss B had the ''necessary mental capacity to give consent or to refuse consent to life-sustaining medical treatment''.

The Department of Health, in a statement, said: ''Miss B has chosen to have all artificial ventilation which she is receiving as part of her medical treatment withdrawn.

''She has died peacefully in her sleep after being taken off the ventilator at her request.''

She died last Wednesday, but the announcement was not made until yesterday - the day Diane Pretty, who is in the advanced stages of motor neurone disease, lost her final legal bid to enable her husband to help her die without fear of prosecution.

Like Miss B, she is paralysed from the neck down, but unlike Miss B would need her husband to help her commit suicide.

Miss B, a 43-year-old former social care professional, argued that she should be allowed to decide whether the ventilator keeping her alive should be switched off, rather than doctors.

Dame Elizabeth agreed. She gave Miss B the right to be transferred to another hospital and be treated in accordance with her wishes, including drug treatment and care to ''ease her suffering and permit her life to end peacefully and with dignity''.

Campaigners were not surprised by the judgment, which they said was merely a restatement of existing principles.

In contrast, helping Mrs Pretty to take her own life is not allowed. The law makes a difference between active and passive assistance.

But it is an arbitrary distinction according to Sheila McLean, professor of medical law and ethics at Glasgow University.

''In Miss B's case you have a woman receiving treatment who has the right competently to reject it and therefore commit suicide with the assistance of the doctors who are obliged to withdraw treatment.

''The law says that is not really assisting in a death, but I would take issue with that. If you have a pre-existing duty of care there is no difference between the culpability for your acts and your culpability for your omissions.''

In other words, both women needed assistance to die, whether by a doctor turning off a life support machine or by a husband administering drugs.

However, Mario Conti, Archbishop of Glasgow, said the distinction was important.

''No-one can fail to be moved by the suffering of Diane Pretty and the immense courage she has shown in taking her case to court. However, no linkage to the Miss B case is warranted,'' he said.

''The principle is quite clear. We may never do anything deliberately which may bring about the death of another.

''Nor may we refuse to give or withdraw basic nursing care in any circumstances. This would be to kill by omission.

''There is, however, no obligation to accept for oneself, nor force upon another, untherapeutic medical care which is burdensome or merely extends life artificially.''

In comparison, he said that Mrs Pretty's wishes amounted to ''deliberate suicide''.

''It is this which the European court has rightly judged to be inadmissible,'' concluded Archbishop Conti.

The distinction is also supported by doctors.

Michael Wilks, chairman of the British Medical Association's ethics committee, said doctors backed the status quo last time they were consulted.

''Overwhelmingly, BMA members from a wide range of moral viewpoints agreed that they could not recommend a change in the law to allow euthanasia and physician-assisted suicide,'' he said.

However, the BMA's ethical guidelines state that ''a voluntary refusal of life-prolonging treatment by a competent adult must be respected''.

In her judgment last month, Dame Elizabeth paid tribute to Miss B, whom she had met at her bedside in a London hospital: ''I would like to add how impressed I am with her as a person, with the great courage, strength of will, and determination she has shown in the last year, with her sense of humour, and her understanding of the dilemma she has posed to the hospital.

''She is clearly a splendid person and it is tragic that someone of her ability has been struck down so cruelly. I hope she will forgive me for saying, diffidently, that if she did reconsider her decision, she would have a lot to offer the community at large.''