Marriage is going out of fashion. Four out of 10 married couples in Britain end up divorced and almost a third of children are born outside wedlock, fairly convincing evidence that something is badly wrong with the institution. Yet we've just been asked to celebrate National Marriage Week, a scheme dreamt up by Richard Kane, director of a little-known charity. It's certainly generated publicity for him and his organisation, Marriage Resource, but what effect has it had on the rest of us? All we know is that, in spite of the grim statistics, Kane would like more people to trip down the aisle or visit their local register office.

This will be music to the ears of the Lord Chancellor, Lord Mackay, who presided over last year's uneasy passage through both Houses of Parliament of a new divorce law making it slightly more difficult, in terms of qualifying periods at least, to get out of a miserable marriage. But it doesn't seem likely that many couples, dithering over whether or not to get hitched, will heed Kane's advice and name the day.

Equally unconvincing is Lord Mackay's insistence that "we need to promote a vision of marriage that is first and foremost positive". These are predictable sentiments but the sudden surge of enthusiasm for marriage this week includes support from an unexpected quarter - the fashionable think-tank Demos, which is widely believed to reflect the views of the Labour leader, Tony Blair. Demos has just published a pamphlet whose author, Helen Wilkinson, admits that marriage is in a sorry state. Yet she argues that what it needs is "to be brought up to date, liberated from the grasp of church and state, and given to the people". The trouble is that the people, at present, are decidedly unsure whether they want it.

The number of divorces in Britain has risen inexorably from an insignificant 6,092 in 1938 to an annual figure in excess of 150,000 in the 1990s. (In 1994, the latest year for which figures are available, it stood at 158,200.) At the same time, there has been a corresponding drop in the number of marriages - in 1993, there were fewer than 300,000 for the first time in 40 years. In 1994, it was down to 291,000, compared with more than 400,000 in 1971- a drop of 28 per cent in just over 20 years.

This means that for every two British couples promising to stay together until one of them dies, another is divorcing - a situation which the Church of England, Lord Mackay, Conservative MPs and now Demos have come to regard as a crisis. The change is generational - in the 55-64 age group, nearly 80 per cent of women are married, while for women in their late twenties and early thirties, it is only 60 per cent. Significantly, another 12 per cent in this age group are living with partners to whom they are not married.

Lord Mackay's solution is simple - urge people to stay married and make it slightly harder to divorce. The muddle the Government is in can be gauged by the preamble to last year's coyly titled - since it was about divorce - Family Law Bill: "The institution of marriage should be supported. Where a marriage has broken down the married couple should be encouraged to take all practical steps to save the marriage."

What this fails to recognise is that divorce is usually the final step in a long, frequently acrimonious process; by the time someone sues for divorce, the marriage is in effect over. Demos, by contrast, has come up with a utopian vision in which weddings are democratised by allowing a much wider choice of venues and celebrants and a system of time-limited marriages, particularly for couples without children for whom Wilkinson suggests a 10-year contract.

There is a slightly New Age feel to all this, especially when Wilkinson protests that you cannot hold your wedding "on your favourite hill or beach, nor can you marry in the place you first consummated your love" - reasonable enough, I suppose, unless it happened to be in the back of a Vauxhall Viva. Demos's support for a more liberal, renegotiable form of marriage represents something of a volte face for left intellectuals who have not historically been keen on the institution. The feminist writer Mary Wollstonecraft characterised it as legalised prostitution while Sidney Webb, who with his wife Beatrice founded the Fabian Society, dismissed it as "the waste-paper basket of the emotions".

The historian Lawrence Stone argues that well into the 19th century a married woman "was the nearest approximation in free society to a slave". Her husband "could use her sexually as and when he wished, and beat her (within reason) or confine her for disobedience to any orders". Professor Stone's work emphasises the extent to which marriage law in England and Wales, from the passage of the Hardwicke Marriage Act in 1753, was an attempt to regulate the passage of property and guard against "spurious issue" - men's perennial fear that their sons and heirs are not their own.

Unlike Lord Mackay's approach, the Demos pamphlet makes a timid attempt to grapple with the real problem - the fact that, for most people under the age of 50, lifelong unions with a single partner have been replaced by serial relationships, often monogamous in themselves but without the "till death us do part" commitment. But it does not come up with cogent reasons why we need marriage at all.

Evidence about the effect of marriage on health and happiness is contradictory, with some support for the notion that men get more out of it than women - which may explain why the vast majority of divorces in Britain are initiated by wives. And while Demos claims that "cohabiting relationships are four times more vulnerable to relationship breakdown than married couples", this may simply represent a greater tendency towards experimentation before non-marrying couples settle down.

The real question is whether so prescriptive an institution, and one which until very recently placed women at a severe disadvantage, can be reformed and brought up to date. And if it cannot, the problem is not so much what happens to adults when cohabiting relationships break up as the effect on children. If, as seems likely, many people are going to spend their lives with two or three different partners, how do we ensure that the interests of their offspring are protected, financially and emotionally? lt is worth pointing out here that the notion that Britain was once a stable society in which married couples lived together in harmony for 40 or 50 years is largely a myth. Divorce has simply replaced death as the prime disrupting factor in marriages - it is no accident that the latter half of the 19th century, when divorce was difficult to obtain and socially disastrous, was the heyday of wife and husband-poisoning. Lawrence Stone insists that "lamentations over the collapse of the family in England are exaggerated, based on a failure to realise that in the past death was as important a cause of the premature dissolution of marriage as divorce is today."

What we do not seem to have recognised is the cost of having serial families. The much-maligned Child Support Agency currently has a live caseload of 1.8 million cases. A recent scan of the CSA's computer system suggested that a huge majority of absent parents - roughly 95 per cent - are men, and other statistics show that the average amount they are asked to pay in child support is pounds 25 a week. Yet the agency has had to institute specialist searches in just under 133,000 cases, and introduce a system of paternity tests where men deny being the father of the child in question. (So far, 17 out of 20 men who dispute paternity have turned out to be lying.)

This is a miserable state of affairs and a more obvious candidate for action than trivial questions such as where couples are allowed to get married. Yet with a government and opposition almost equally committed to "family values", we shall probably go on inventing pointless wheezes like National Marriage Week for years to come. Meanwhile, many of us will continue to enjoy unregulated unions which we can enter and leave at will - in effect, a parallel system over which the state, because it is reluctant to recognise its existence, has almost no control. If love and marriage ever did go together like a horse and carriage, this particular horse has long since bolted.

'Proposal: Giving Marriage Back to the People' by Helen Wilkinson with additional research by Alison Beeney, is published by Demos, pounds 4.95.