The number of breadwinning mothers, which includes those earning more than their partner or those who are working single mothers, has soared from 18 per cent 15 years ago to 31 per cent today, the study by the IPPR think tank found.
More than 2.2 million working mothers are the main earners in their families, an increase of one million, or 83 per cent, since 1997.
When child benefit and other measures are taken into account, working mothers contribute more than half of all earnings to family budgets.
The rise has been driven by a steady increase in women's opportunities in the workplace, coupled with a steady decline in traditionally male jobs in manufacturing. Economic austerity has also driven single mothers who previously did not work to look for employment, researchers say.
While the findings will be celebrated as a key step in female equality, the report's authors said the Government and employers must respond with greater flexibility to working women, who could still end up doing more childcare than their partners outside of their jobs. Firms should recognise the growing economic force of breadwinning mothers and offer more flexible hours, while the Government should provide more affordable childcare to take the pressure off working women.
The IPPR report, which is published to coincide with the end of the Duke of Cambridge's two-week statutory paternity leave, states that the results "demonstrate unequivocally that mothers' incomes are vital to the economic survival and wellbeing of a rising number of families. It is hard to see how this trend could possibly be reversed in the future."
The number of cohabiting mothers who are breadwinners has doubled since 1997 to 200,000, a faster rate than married parents and single working mothers.
However, the employment rate of single mothers has increased from 43 per cent in 1997 to 58 per cent last year, the latest figures that are available.
Graduate mothers are more likely to be breadwinners than those with fewer skills, with 35 per cent of mums with a degree earning more than their partner, compared with just over a quarter for those without this level of qualification.
Despite the apparent cause for celebration, the IPPR report warns that working mothers still only account for a third of breadwinners, and face "significant barriers to entering and remaining in work" – including a lack of flexible work opportunities, the soaring cost of childcare, and the emphasis on maternity leave, rather than both parents sharing childcare. Shared parental leave will be introduced in April 2015.
The barriers disproportionately affect mothers, because working women usually take primary responsibility for care. The gender pay gap for women over 30 remains large, although this is not so for men and women in their twenties
The IPPR called on the Government to introduce affordable universal childcare to allow families to balance their responsibilities in response to the growing economic power of working women.
The report, which is part of a wider project by the IPPR on the "Condition of Britain", also points out that the increasing number of working mothers can put additional pressure on family time and on "maintaining stable, quality relationships". It suggests that Britain could develop along the lines of the German Familienpflegezeit model, or "family caring time", which allows employees, male or female, to reduce their working time and income over a fixed period to care for a child or other dependents. When employees return to full-time work, they continue to receive a reduced income to refund the difference.
In Iceland, families are entitled to five months of maternity leave, followed by five months of paternity leave, and finally a further two months of parental leave, which couples can decide how to use.
The rise in breadwinning mums also reflects the changing view of a woman's role. The British Social Attitudes survey reveals that, in 1986, 46 per cent of women believed it was a "man's job to earn money and a woman's job to look after the home". In 2006, this had fallen to 15 per cent.
Dalia Ben-Galim, IPPR's associate director and one of the authors, said: "The balance between breadwinning and caring has changed; it can no longer be assumed that the dad is the primary breadwinner in a couple family. But despite more mothers than ever before being the primary breadwinner, many mums still face significant barriers to entering and remaining in work, such as a lack of flexible work opportunities,the high cost of childcare and parental leave entitlement focused on mothers."
The report was sponsored by the relationship service Relate, whose chief executive, Ruth Sutherland, said: "Government policy is not keeping up with what is now the norm for many British families. The inflexibility of parental leave and the high cost of childcare are making it more difficult for families to decide what works best for them, in terms of who goes out to work and who cares for children."
Case studies
'He's quite happy to take the kids to parties with the other mums'
For Gemma and Barry Cole, deciding who would be the breadwinner after they had children was easy. With two kids, aged four and two, the pair from Cockermouth, Cumbria are part of an increasingly popular family set-up where fathers stay home. Gemma, 32, returned to her job at her father's motoring firm after maternity leave for each birth. Barry, 35, gave up his career as a roofing contractor to raise their children, briefly returning to it while his wife was on leave.
"It was always the decision before we even had a family," says Gemma. "It made more sense [for me] to stay in the family business."
The couple were never at odds with swapping traditional parental roles. "He's quite happy to meet with the girls, and take the children to birthday parties even though it's all mums there."
Going against the grain won't affect the couple's decision to have more children, though it can be tricky at times, Gemma says. "It's probably harder the second time as you've done it once and [going back to work] broke your heart." She says the children are happy with the arrangement, although their daughter often calls her Dad, which was hard to get used to.
The couple were surprised by the lack of provision for stay-at-home dads, particularly given the percentage of maternal breadwinners in the North-east and North-west.
Kashmira Gander
'I wanted something new. My husband is happy at home'
Angela Lyons-Redman, 44, has two daughters, aged 10 and eight. She runs her own plumbing firm and lives in Chiswick, west London. Her second husband is a graphic designer and works from home.
"After I had my children," Angela says, "I wanted to do something new. I noticed how difficult it was to get a good plumber, so I decided to set up my own business, which now employs 20 people. My husband is happy working from home.
"I know several women, most of my friends, who are the main breadwinner, so I have noticed this trend. Some of the bigger forward-thinking firms are recognising the changing trends and supporting women more so they have the chance of a career and to have a family. It would be great if every company had a crèche, for example."
Jane Merrick
