Marriage is back in fashion with three out of four saying they'll do it - one day

Despite figures suggesting the institution is in terminal decline, three out of four young cohabitees say they will tie the knot one day.

The findings add to growing evidence that tax reforms which make it financially beneficial to stay single are to blame for the falling number of weddings.

It was revealed last week that only 12 out of every 1,000 single people married in 2005, the lowest rate on record.

The latest survey does not examine in depth the reasons that people appear to be waiting longer before marrying. But it did find that many mothers have identified the financial advantages.

Researcher Dr Ernestina Coast, of the London School of Economics, examined the views of cohabiting couples under the age of 35 as given in the British Household Panel Survey, a database which has traced the lives of more than 10,000 people since the early 1990s.

"When questioned about their future intentions, three quarters of men and women reported that they were planning to, or probably would, get married," said Dr Coast. She said the majority said there were no positive advantages to living together rather than marrying. But many of that majority were childless couples. Her analysis found a major difference in the views of those who had children.

"When questioned about the reasons for viewing cohabitation as being advantageous compared with marriage, mothers are significantly more likely to report financial advantages than fathers," she said.

"Childless women are significantly more likely to see cohabitation as a trial marriage than cohabiting women who are already mothers."

Former Labour welfare minister Frank Field said last month that tax credits operate "brutal discrimination" against couples.

Under the system, only one working parent gets benefits, and nothing is paid to help a second adult in a home.

Mr Field found that a single mother with two children under 11 on the minimum wage receives tax credits that take her weekly income to £487 if she works only 16 hours a week.

But a two-parent family with one earner would have to put in 116 hours of work to get the same money.

Whitehall figures say that around two million people are currently "living apart together", a status that means they consider themselves a couple but maintain separate addresses. Tax credits are thought to be a reason for the popularity of such living arrangements.

The Government has stripped away many of the privileges of marriage since 1997.

The last tax break for husbands and wives, the Married Couples' Allowance, was abolished. Legal rights once reserved for marriage have now been extended to gays under the civil partnership system, and are likely to be given to some unmarried couples in the near future.

Nevertheless, Dr Coast's study, funded by the Government's Economic and Social Research Council, holds out some hope for the future of marriage.

Women are still the biggest losers from divorce, researchers have said. Their income falls by 17 per cent after a break-up while men see theirs go up by 11 per cent.

The study suggested most divorced women would be better off financially finding a new husband rather than trying to further their careers.

A series of high-profile divorce settlements recently have suggested women are benefiting from increasingly large payouts.

But the Belgian research suggested otherwise. The people who do worst from divorce are parents, they said. And in most cases that means the mother, as women are most likely to take the children, leaving them less free to pursue a career.

Mieke Jansen, who led the survey, said: "The presence of children has a negative influence on income, and this effect becomes stronger as the number of children increases.

"Having young children makes repartnering much more effective than entering the labour market or increasing working hours."