The number of female MPs could plummet for the first time in more than a century if action is not taken now, campaigners have warned.
If Reform UK, which has led voting intention polls since April, or the Conservatives, win the next general election, women's representation would fall drastically.
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A Labour win would stall the current percentage of female MPs, which stands at its highest ever at 41% (264 out of 650) - but is still lower than the UK population, of which 51% are women.
If Reform win the next election, set for 2029, women's representation could fall to 26%, analysis of Electoral Calculus polling data by 50:50 Parliament found.
If the Tories win, 33% of MPs would be women, while Labour would remain the same.
What does history tell us?
Every parliament since women were allowed to be MPs in 1918 has seen an increase, apart from small drops in 1950, 1979 and 2001.
Nancy Astor was the first woman to take her seat in the Commons and served from 1919 to 1945.
The 80s saw a substantial increase in female MPs before a large jump in 1997, and there has been a big rise from 128 women MPs in 2005 to the current 264.
Despite this, no major party reached gender parity in its selection of candidates for last year's election.
Labour selected 47% women, Greens were at 44%, Conservatives 34%, Lib Dems 28% and Reform 16%.
50:50 Parliament, which campaigns for gender equality in politics, is calling for people to "Ask Her to Stand" to encourage more women to get involved with politics to help balance the scales.
It has installed a "push for equality" panic button outside parliament to raise awareness of the gender disparity.
Stella Creasy, Labour MP for Walthamstow since 2010, has campaigned for abortion rights, childcare reform, and became the first MP to appoint a locum MP to manage constituency work during her maternity leave, after campaigning for better maternity rights for MPs.
She told Sky News gender parity in parliament is important, as a broad range of views and experiences is needed "to shape good quality policymaking".
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"For me, this is not about electing women for the sake of it, or because they champion women's rights per se; it is because gender balance will lead to better outcomes for all and the furthering of all of our rights," she said.
"There is strength in diversity, and we cannot hope to make decent policy, or pass good legislation, if half of the population are not adequately represented in the House.
"Childcare is a perfect example - it isn't just good for mums, it is essential infrastructure for society and therefore the economy to get it right."
'Why would you think men are better at politics?'
Harriet Harman, a Labour peer and co-host of Sky News' Electoral Dysfunction podcast, is a former minister who put forward the Equality Bill, now the Equality Act 2010, which protects people from discrimination in the workplace and in wider society.
She told Sky News: "The time is long past that women could be expected to put up with men making all the decisions.
"Male-dominated parliaments are evidence of discrimination. Unless you think men are just better at politics than women. And why would you think that?
"A team of men and women with a breadth of experience is what works best. Who wants to go back to the old boys' network running the country? Not me."
'Different voices make better decisions'
Former leader of the Scottish Conservatives Baroness Ruth Davidson, also a co-host of Electoral Dysfunction, said parliament "works better when it looks and feels closer to the country it seeks to represent".
"Women politicians often have different experiences before entering parliament, including in their prior interactions with public services," she added.
"It is important that laws and funding decided in the Commons and elsewhere are informed by the widest experience of how such decisions impact in practice.
"Having different voices round the table, offering challenge from a variety of perspectives, is how you make better laws, better decisions and a better Britain."
Lyanne Nicholl, CEO of 50:50 Parliament, said the impact of their prediction playing out will be "devastating", as she pointed out gender parity "isn't about party politics - it's about democracy".
She said "we risk turning back the clock" and policymaking "ignoring half the population", as she called the data a "wake-up call" for everyone.
(c) Sky News 2025: Number of female MPs could plummet for first time if no action taken
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