As a psychiatrist working with adults compulsorily detained in hospital, I have four questions about the proposals in the Mental Health Bill. Are they fair? Will they do harm? Will they do good? And what should be the view of civilised people? The answers are alarming.

"The principles governing mental health care should be the same as those that govern physical health," said Professor Genevra Richardson, who led a review of the subject to prepare the way for new legislation.

Her statement sounds fair. Unfortunately the Government chose to ignore it. The physically ill have the right to refuse treatment even if a refusal would lead to injury or premature death – but if this Bill is passed there will be no such right for the mentally ill.

Suppose I have two patients, one with schizophrenia and one with cancer. Both patients recognise that they are ill, that their illnesses can be treated and that there would be consequences to not receiving treatment. They each have the same level of understanding of their illness and the proposed treatment (in legal terms they are both capable).

The patient with cancer may refuse my treatment, and if I go ahead with it I will be committing an assault. But I will have a legal duty to impose treatment on the patient with schizophrenia.

The patient with cancer is also able to make a legally binding advance directive declining particular medical treatment of the illness for a future time when he loses capacity. That possibility will be denied the patient with schizophrenia. If my patient has both conditions he will be able to refuse treatment for the cancer but not for the schizophrenia. This is unfair, absurd and makes the mentally ill lesser citizens.

There are two main reasons why some mentally ill patients who have capacity refuse treatment. First, they may believe it is not suitable for them, even though a doctor has explained the disorder, the treatment, the consequences and so on. Second, they may believe the side effects to be worse than the illness. Physically sick patients have their views respected, but not so the mentally ill.

The proposals are also unfair when it comes to detention. There is a long history of detaining the mentally disordered lest they do something wrong. The Criminal Lunatics Act of 1800 provided, for the first time, for detention "at His Majesty's pleasure". It included not only those found guilty of a crime while insane but also any person "discovered or apprehended under circumstances that denote a derangement of mind and a purpose of committing some crime".

That means preventative detention, or internment as it was called in Northern Ireland – the indefinite detention of a person because someone else thinks he or she might be a danger, despite having committed no crime.

Let me describe life as a detained patient. I speak here with many years' experience as a Mental Health Act Commissioner, in addition to work as a consultant psychiatrist. It commonly means living in wards that are inadequately furnished and maintained, with too many patients, too much disturbance and too few staff, many of whom have inadequate training. It means having little or no say in which treatments you will receive. It leads to many losses, including loss of fresh air, exercise, choice of bedtime, choice to watch what you want on television, choice to drink alcohol or make love. It leads to profound boredom, with little daytime activity except smoking.

The Government says it is now going to invest heavily in mental health services to make them relevant and attractive, with plenty of well-trained staff. If true it is hard to understand why we will need more compulsion.

You might think that I, as a psychiatrist, could ensure that only truly dangerous people are locked up. Unfortunately our ability to predict serious violence is poor. Over the past 10 years we have had a large increase in the number of people detained. This has been driven by the culture of blame and consequent fear among psychiatrists as a result of unrealistic expectations of our ability to make the world safe. We would need to detain between 2,000 and 5,000 people, unnecessarily, to prevent a single killing.

Will the proposals do good? The best way to protect the public is to try and ensure that those who might harm others due to mental disorder present themselves for assessment and, where appropriate, treatment. I struggle to believe that my acceptability to patients will be improved if they know that I will have a duty to enforce treatment on them, not only when they are poorly but also when they are well (to prevent a possible relapse), both in hospital and in the community.

Talking of enforced treatment in the community, how is this to be delivered? The image of me sending the police or community nurses to a patient's home to drag them to hospital for an injection is not one that will, in my opinion, enhance the likelihood of patients turning to me for help at a time of distress. I am also far from convinced that illicit drug users will rush forward for assistance knowing that I will have a legal duty to report their criminal behaviour to the police.

So to my last question: what is the civilised view? The High Court judge Dame Elizabeth Butler-Sloss said during the recent case of Ms B, a physically ill patient who wanted to die: "A competent patient has an absolute right to refuse to consent to medical treatment for any reason, rational or irrational, or for no reason at all, even when that decision may lead to his or her death."

If only that were true for the mentally ill, as it is for everyone else. It is time for all those who care about justice to campaign for that end. All patients who can understand what is happening to them and are able to make decisions about their own treatment should have the right to refuse it. Anything else, such as the measures proposed in this bill, is discriminatory, stigmatising, and doomed to failure, for patients and non-patients alike.