The traditional nuclear family has altered radically in the last 20 years, a national report published today by a Bradford academic claims.

The latest annual British Social Attitudes survey identifies a huge shift in attitudes to family life over the past two decades.

New Families? Tradition and Change in Modern Relationships, by Simon Duncan, Professor of Comparative Social Policy at the University of Bradford and Miranda Phillips, of the National Centre for Social Research, claims the heterosexual married couple is no longer the "norm" in British society.

Instead, 70 per cent of people think there is nothing wrong with sex before marriage, compared with only 48 per cent in 1984.

Some two-thirds of people (66 per cent) believe there is little difference, socially, between being married and living together, and only one in four - 28 per cent - think married couples make better parents than unmarried ones.

However, views remain more traditional when children enter the picture.

Prof Duncan said: "The heterosexual married couple is no longer central as a social norm, but views are more traditional when it comes to bringing up children.

"Children seem to hold a particular position in people's attitudes to family life. When they are involved, alternative family arrangements are seen as less acceptable."

Only four in ten people (42 per cent) think one parent can bring up a child as well as two. The same percentage also disagree with the view that a gay male couple are as capable of being good parents as heterosexual peers.

And although nine out of every ten people think donor insemination should be allowed for a couple who cannot have children naturally, the figure falls to six in ten - or 61 per cent - in the case of a single woman.

The number of single-person households has increased, co-habitation has increased and marriage rates remain at their lowest since 1986, the report adds.

It also identifies a little-publicised trend - men's views on marriage and parenting are becoming more traditional than women's. A third of men (34 per cent) believe married couples make better parents than unmarried ones, compared with only 23 per cent of women.

Dawn Catley, centre manager of Bradford lone parent support group Gingerbread, said: "As a society we are more accepting of individuals' personal choices with regard to relationships and marriage. From working with lone parents over the last 17 years I have seen lone parent families become less stigmatised.

"However, lone parent families take an unfair proportion of the blame for problems in society and especially children's bad behaviour."

The survey claims the majority of people now see unconventional family set-ups as a natural part of everyday life.

Some 61 per cent are now of the view that single women should be allowed to use donor sperm to become pregnant.

And only 18 per cent of survey respondents took the view that homosexual relationships were always wrong.

"There always is change," added Prof Duncan. "The so-called traditional family of the 1950s was always a very short and rare thing. Maybe the current family form is going back to what was recognised as the norm in the 19th century."