Women. Like men, only cheaper. And in 2018 we are discovering by just how much. The introduction of mandatory reporting has transformed the debate about the gender pay gap from a little-discussed dry set of statistics directly into staffrooms and workplaces across Britain, with explosive effect. The data shows in black and white just how few women are progressing within organisations across every area of the economy – whether in the public or private sector, gender still too often predicts pay.

Yet even before all these reports have been made public, the backlash against any such scrutiny has begun. Angry voices decry the gender pay gap, saying it doesn’t exist, or that we’ve got it muddled with equal pay – something entirely different. Commentators such as Jordan Peterson, invited on to BBC Radio 4’s Today programme to discuss the gender pay gap on the day of the deadline for companies to report their results, claim women don’t have the skills to be top executives and don’t work the long hours men do, so the data is meaningless. Others say the gap just reflects life choices women have made; the “mummy pay penalty” for women who take breaks to have children or want to work part-time. Women, know your limits – and be paid accordingly.

It’s telling that these pundits provide no evidence themselves to back up their prejudices – because the data there is proves their analysis is opinion, not objective fact. As the Office for National Statistics points out, women taking time out for childcare only explains some of the variation in pay. Only 36% of the gender pay gap could be attributed to specific characteristics such as differences in average age, job tenure, company size and occupation. Talk of the lack of women in comparable roles also misses the point that women aren’t in those roles to compare – it’s not that women are incapable of being top executives or working all the hours in the day, it’s that still too often they don’t get appointed to do so.

These ideologically suspect attacks confuse causation with correlation; they reinforce social preconceptions that it is women who should sacrifice their careers if they start a family. It’s a view that dominates discussions about flexible pay, and in turn the persistent undervaluing of part-time work – because that’s for those who have “given up” ambitions for promotion and are earning pin money.

Patriarchy has held us all back for too long with such rhetoric – tired old notions that there are boys’ jobs and girls’ jobs, and that gender dictates ability. Ryanair defends having a gender pay gap of 72% by pointing out how few women are pilots and how many are cabin crew. Their chief executive claims it is a “feature” of the airline industry, , as though a penis is required to be able to fly a plane. Yet even such biologically determinate arguments are blown apart by this data. In the NHS where 77% of the workforce is female, women still only account for a minority of senior leaders. It also doesn’t explain why even in companies such as Marks & Spencer, where there is a smaller pay gap, the bonuses paid to women are still substantially lower.

Quick Guide What is the gender pay gap, and what must UK companies report? Show What is the gender pay gap? The gender pay gap is the difference between the average hourly earnings of men and women. The figure is expressed as a proportion of men’s earnings. According to the ONS, the gap between what UK male and female workers earn – based on median hourly earnings for all workers – was 17.9% in April 2018, down from 18.4% in April 2017. Data in 2018 showed that men were paid more than women in 7,795 out of 10,016 companies and public bodies in Britain. What is being published? All companies and some public sector bodies in Great Britain, except Northern Ireland, with more than 250 employees had to report their gender pay gap to the Government Equalities Office for the first time by by 4 April 2018. The second year of gender pay gap reports - and the first indicator of how public bodies and companies are performing - must be filed by April 2019 What’s the difference between the mean and the median figures? Commonly known as the average, the mean is calculated by adding up the wages of all employees and dividing that figure by the number of employees. The mean gender pay gap is the difference between mean male pay and mean female pay. The median gap is the difference between the employee in the middle of the range of male wages and the employee in the middle of the range of female wages. Typically the median is the more representative figure, because the mean can be skewed by a handful of highly paid employees. What will happen if companies don’t report? The Equality and Human Rights Commission (EHRC) said that, while it would approach employers informally at first if they failed to publish figures by the deadline, businesses could ultimately face “unlimited fines and convictions”. However, information published following a freedom of information request by the Guardian showed that no companies have been fined to date despite hundreds failing to accurately file their gender pay gap figures on time.

What these arguments do prove is that tackling the gender pay gap isn’t just about ensuring that access to shared leave, flexible working and job sharing is standard and used by both parents. It’s also about employers and their responsibilities when it comes to recruiting, retaining and promoting women in the first place. Responsibilities that, if ignored, manifest themselves in pay. Research shows women ask as often as men for a pay rise – yet men are four times more likely to get one. That women exhibiting the same leadership skills as men are judged completely differently. Responsibilities that start early too: a year after graduation women already earn on average £1,600 a year less than their male counterparts.

In the fight for equality we must not let such voices gaslight women that the gender pay gap is a natural phenomenon. Instead we must use the data to drive action to bust open such stereotypes about what women are capable of and what kind of work/life balance every employee needs. Yet there are worrying reports already of women being told to “raise a grievance with HR” if they ask their managers about this data, or that talking about it could make them be seen as troublemakers. That’s why we started the “#PayMeToo” campaign – to give practical advice on how to talk about it at work and what rights employees have to do so, and show that MPs are ready to act if this conversation gets shut down.

We know that this is not the full story on inequality in the workplace – with some companies not including partnership data, which is not mandatory, and the sole focus being gender and not ethnicity, class or disability. But we also know this data is telling us what James Brown sang – that it’s still a man’s world. #PayMeToo is about reminding us that it would be nothing without a woman or a girl.

• Stella Creasy is Labour MP for Walthamstow