Women will have shouldered 85% of the burden of the government’s changes to the tax and benefits system by 2020, according to figures published ahead of next week’s autumn statement.

The analysis, by independent thinktank the Women’s Budget Group, shows that tax and benefit changes since 2010 will have hit women’s incomes twice as hard as men by 2020. Women will be £1,003 a year worse off by 2020 on average; for men, this figure is £555.

Less affluent women will be the worst affected: those with below-average incomes will find themselves £1,678 worse off. Many belong to the “just managing” families – those getting by on less than average earnings – that Theresa May has pledged will be a key priority for the government.

Equal Pay Day: how the UK's gender gap in earnings has shifted over the years Read more

The Women’s Budget Group’s forecasts assume that the government proceeds with all of its planned changes to tax and benefits in the runup to 2020, including the roll-out of universal credit. They also take into account increases in the “national living wage”, set to rise to £9 an hour by 2020. Eva Neitzert, the group’s director, said: “Women, especially those on low incomes, have shouldered the largest burden of austerity measures. Overall, women stand to lose twice as much as men by 2020, and for those on the lowest incomes this means a cut of between 11 and 15%.”

In her first speech as prime minister, May committed her government to fighting against “burning injustice”, including addressing the gender pay gap. As home secretary, she won plaudits for her work to tackle gender issues such as domestic violence, female genital mutilation and forced marriage.

But the latest analysis shows how far measures introduced since 2010 by the former chancellor George Osborne have increased gender inequality. In particular, universal credit, a new benefit that merges several benefits and tax credits and will be phased in over the next few years, will be paid by default into the account of the main earner in each household – unlike the benefits it replaces. Women’s groups have argued this will undermine women’s economic independence.

“The prime minister deserves credit for her work as home secretary in tackling some gender issues such as violence against women,” said Vivienne Hayes, chief executive of the Women’s Resource Centre, an umbrella body for women’s charities. “But these figures that reveal women are bearing over 80% of the burden of cuts to tax credits and benefits highlight that she will need to raise her game in No 10 if women are to really believe she cares about tackling women’s inequality.”

Lone parents – 90% of whom are single mothers – will be particularly affected. Some working single parents will lose up to a month’s income as a result of cuts arising from universal credit, says the charity Gingerbread.

Tracey is a single parent in her late 30s, unable to work because of disability. Over the past six years she has experienced cuts to disability benefits, housing benefit and council tax benefit that have left her struggling with living costs. “I’ve cut everything back as much as I can. I don’t smoke, I don’t drink, I don’t go out any more,” she said.

“I often have to put off paying one bill in order to keep the heating on. I sometimes end up needing to borrow from friends without knowing when I can pay them back, which I hate doing.”

The government has claimed the national living wage, introduced in April this year, will provide a boost to lower-earning families. But for many families any positive impact has been more than outweighed by cuts to tax credits.

“The introduction of the national living wage has been a good thing for women, who make up 60% of the low-paid workforce,” said David Finch, senior economic analyst at thinktank the Resolution Foundation. “But the good news on pay simply cannot compensate for such large welfare cuts, especially among women who are disproportionally worse off.”

The new figures come in the wake of a report on Friday from the women and equalities select committee that called for an independent inquiry into why the government has not published a proper analysis of how its spending plans will affect women, minorities and disabled people.

“This begs an obvious question: are Treasury ministers deliberately seeking to hide inconvenient truths about the impact of its policies on women?” said Neitzert. “The Treasury has a legal obligation to have due regard for the impact of its policies on equality but doesn’t seem keen to do this in any meaningful way.”