Most religions have a day of the dead, and today is that day for Christians. In this most secular nation, polls show few now know what Good Friday is for, this day for sorrow, for contemplating death, loss and endings.

But here the usefulness of faith ends, for it is mainly the power of the religious lobby that forces people to die in pain and indignity due to beliefs on the nature of life and death shared by very few. For 20 years now, every poll on the subject shows that 80% of people want the right to be helped to die at a time and in a way of their own choosing. But that kind of "choice" is not on the agenda. Or not yet.

It happens to be a good day for contemplating how we die and watch others die as the US courts finally let Terri Schiavo go. She has been 15 years a-dying in a persistent vegetative state, probably beyond pain, though not beyond reflex responses. But if there is still suffering to be had, now in her seventh slow day without water or food, the law inflicts death by slow dehydration in the name of "ethics". It's a shocking spectacle that could be stopped with one merciful injection. But here in our own dying rooms similar terrible ethical deaths are inflicted on British citizens every day by kindly nurses and doctors. There is a conspiracy of silence about the actual processes of death.

What kills you in the end if you have cancer or other terminal diseases? Not often the cancer itself. Nor the morphine that people innocently imagine will one day waft them away on a cloudy pillow of dreams to some opium-fuelled nirvana. What people actually die of, like Terri Schiavo, is dehydration when they can no longer swallow enough water to live - and it takes time. Enough morphine to die quickly is very rarely administered these days. Instead, cautious doctors, extra wary after Harold Shipman, give just enough morphine to kill people by degrees. It is enough, in the very end, to render them unable to drink so they die, semi-conscious, of thirst. Hospices don't put up drips to keep people alive, but they don't give out death-dealing injections either. The legal compromise is death by dehydration or sometimes slowly and gasping for breath by morphine-induced chest infection - "old man's friend". That is the great unspoken truth.

The other unspoken truth is that morphine - the only existing, effective pain-relief - is a cruel drug whose main side-effect is such acute constipation that final months are spent in far greater agony from cemented bowels punctuated with bursts of uncontrollable diarrhoea than from the original disease. The indignity of bowel-obsession is no way to end up. Nor does morphine ease anxiety: on the contrary, frightening hallucinations are frequent so patients need anti-depressants as well, but those have their own side-effects. Morphine causes nausea, requiring anti-sickness medication. Nor does it take the away the terminal sense of a body crumbling and falling apart.

Many, like my mother, thought signing a living will to refuse intrusive treatment would be enough. But she, like so many others, was left begging everyone and anyone for a quick release from an intolerably drawn-out, inevitable end. It is one of the law's cruelties that a healthy person can kill themselves or an irrational person can deny themselves life-saving treatment, while the terminally ill close to death are forced to linger on beyond what they can bear.

Since I wrote about my mother's unkindly death, I have been inundated with similar stories. People often come up to me and pour out their tales of miserable deaths of horror, indignity and pain. The shock of discovering the truth about death fills people with indignation. They feel they must tell others, must give witness to what happens, aghast that in the great cacophony of heated debates about insignificant things, this torture still goes unremarked. Consider those who have operations while not properly anaesthetised. What happens in death may be as bad or, for all we know, worse, since no traveller returns from that bourn to tell us how it felt.

Perhaps people prefer a sanitised myth about modern dying. They hear the falsely reassuring murmurs from the religious and the BMA who claim that the best palliative care these days can take away pain and anxiety: no need for euthanasia. But in polls most doctors disagree with their own trade union. Over 80% of churchgoers, Catholic and Protestant, disagree with their church leaders. Nearly half the population say they would even go so far as to break the law and help a dying relative to die. (Though when faced with the prospect, most, like me, would probably be too cowardly to do it).

Good though palliative care can be - my mother had the NHS at its very best - its own practitioners admit they often watch people die in great mental and physical anguish. People clutch at doctors' sleeves, begging for an injection: "Can't you do something?" How easy it is to slip into death-like unconsciousness under an anaesthetic, gone into oblivion before you can count to five. That little death in the operating anteroom is a paradigm for how the good death could be for those who want it. Why not, the dying ask?

On April 4 the argument may progress with the publication of the report from the Lords committee on the Joffe bill on assisted dying for the terminally ill. Rumour suggests that some on this finely balanced committee, who had been opposed, were deeply moved by the evidence from 135 witnesses, thousands of submissions and their visits to the Netherlands and Oregon, where assisted dying is legal. It is rumoured minds were changed when they found their deepest fears unfounded. Numbers choosing assisted death were not rising; palliative care improved as death became an open, transparent business.

Here it is often a furtive affair, with doctors and nurses in fear that some demented relative might accuse them, unjustly, of murder. The committee was shown death as a matter of the best clinical care, a process to be eased when people wanted it - not as the abstract debating-society topic of sound and fury. If the committee's report finds in favour, it could start a huge demand for a change in the law.

As the Pope rasps out his last breaths, his bishops are using his final suffering as a testament to the religious requirement to endure whatever quality of life God sends. Both C of E and Catholic archbishops here will fight any attempt to change the law. Politicians have taken their cue from the churches. Only the Liberal Democrats have a policy to support it (on a free vote).

Before an election, expect no debate on anything as sensitive as this. Elections shrivel debate into an ever-narrower spectrum of issues. Death, and catastrophic climate change, will be airbrushed out. But sooner or later we all must die - and the manner of our going will matter to us very much.

polly.toynbee@theguardian.com