By Hannah Richardson
BBC News education reporter
The role of fathers is to face the greatest change, says the report
The nuclear family is breaking down as children are increasingly raised by relations other than their parents, a government-backed parenting group says.
The Family and Parenting Institute says grandparents, aunts and uncles are helping out more as parents struggle to cope with marital breakdown and work.
One in four children is now brought up in a one-parent household, the vast majority of which are led by mothers.
Soon there would be no typical family model, the institute suggested.
Chief executive of the institute, Dr Katherine Rake, said "families were pulling society in multiple directions, between work and home life, singleness and cohabitation and marriage, between growing older and forming families across our many cultural divides".
Families' adaptability constantly amazed, she said.
But it was fathers who were predicted to lead the coming change, Dr Rake argued.
"The role of fathers is set to change dramatically over the next decades in the way that women's roles have changed since the 1950s."
She said that fathers' role in the family used to be a "very small one".
"This reflected a reality in which women were looking after the homes and mothers were doing all the domestic work and childcare," she said in a speech.
But more women were in work and men were beginning to feel they were not spending enough time with their children and at home.
"Though they still spend much less time actively involved with their children's lives than mothers, fathers are now spending 200% more time 'actively engaged' with their children than in the 1970s," she said.
But she warned that against this backdrop was another trend profoundly affecting fatherhood.
This was that "men are much less likely than in the past 60 years to be living in the same household as their children".
"One quarter of families are headed by a single parents, nine out of 10 of whom are mothers," she said.
She predicted that if these trends continued, fathers would be "pulled in two directions; the desire for greater involvement, but the physical fact of living apart".
At the same time there would be an increase in non-traditional models of family structure and increased reliance on members of the wider family for childcare.
In 2006, two-fifths or 40% of all working mothers relied on informal childcare, with grandparents being the most common source for a third of parents.
It was thus important to recognise the "positive support" of grandparents, but not to exploit them "so that they do not get left 'holding the baby' because they look like a cheap solution when parents run into trouble".
Because of the change ahead for family structures, services would have to be more flexible in response.
"We also have to recognise those families whose lives are 'atypical', who do not fit into trends," she said.
"So while the average age at first birth is going up, teenage pregnancy rate is falling only slowly and teenagers from the most deprived areas have maternity rates 12 times that of the least deprived."
And while mothers' employment is up, single mothers with children under five are the least likely to be working with employment rates 28% lower than for mothers who live as part of a couple.
Shadow education spokesman David Willetts says: "The long-term commitment of two adults to each other so they can raise their children together is not going the way of the bowler hat or Woolworths.
"It is wrong to misinterpret specific social changes such as most women being in paid work and the growing role of grandparents, as meaning that the so-called nuclear family is disappearing when it is not."