If you wondered why a core goal of the Women's Equality Party is equal treatment of women by and in the media, today's Daily Mail answered that question. One of the UK's bestselling newspapers reduced two powerful women to a couple of pairs of 'pins'.

"Never mind Brexit, who won Legs-It!" smirked the headline, next to a picture of the Prime Minister of the United Kingdom and the First Minister of Scotland, who were meeting to discuss critical political issues: The UK's departure from the EU and the prospect of a second independence referendum for Scotland.

Theresa May and Nicola Sturgeon were wearing knee-length skirts as they smiled for the cameras. For Mail columnist Sarah Vine this appeared a "vast expanse on show". What was actually revealed was the Mail's and Vine's own prejudices.

Vine's article focuses entirely on May and Sturgeon's clothes, shoes and body language – and nail varnish. Sturgeon's clasped hands are, we are told, an attempt to "stop herself poking her rival in that gimlet eye", while her "shapely shanks are altogether more flirty, tantalisingly crossed" than May's.

This is a photograph of two women having a business meeting. It reads like a Barbara Cartland novel.

Imagine a piece like this being written about a meeting between two male politicians. Better yet, head to Twitter where some wit has mocked up an alternative front page, featuring the legs of David Cameron, Jeremy Corbyn, Michael Gove and Boris Johnson. Make sure you've finished your dinner first.

What is worse, yet also entirely predictable, is that the Mail engaged a female writer to do the job of denigrating and ridiculing two powerful women, repeating the age-old trick of using a woman to attack other women because if a woman says it, it can't be sexist. Right?

Our co-founder Catherine Mayer took to the airwaves this morning to discuss 'Legs-it!' on BBC Radio 4. Entirely predictably, she was pitted against another woman – Angela Epstein of the Telegraph – who claimed that May and Sturgeon invited, indeed courted, such scrutiny of their wardrobe, and pointed to their appearances in women's fashion magazines to justify this claim.

By this logic, anyone in the public eye who has ever posed in, you know, clothes for a magazine shoot has consigned themselves to being judged first and foremost by what they wear, so that this becomes the context for everything else they ever do. Clearly, this is laughable. (If you listen back to Catherine's Radio 4 appearance, you will indeed be able to hear her laughing quite a bit).

It also smacks of a double standard that is rarely articulated: women in the public eye must validate themselves as 'fashionable' in order to be taken seriously.

Who remembers the abuse eminent academic Mary Beard and feted designer Jenny Beavan endured for daring to dress in way that refused to conform to the media's construction of desirable femininity?

Yet as soon as they do so, they are reduced to nothing more than their clothes and appearance, and their power and legitimacy is eroded. This just does not happen to men.

The press has enormous power to form ideas, and to shape the culture we live in. The Mail's headline today shows us why the battle for equal treatment in the media is so necessary. It is part of a media establishment in which men dominate senior editorial positions and thus literally decide what is news.

The Mail is a repeat offender but it is by no means the only news organisation to casually objectify and sexualise women, and to recognise only a few female archetypes: vamps, victims, blowsy older women.

Its editor (a man) signed off the Legs-it headline. He also responded to the outrage it provoked by changing the front page in later editions to attribute the "lighthearted" piece explicitly to Sarah Vine. This sums up the Mail's attitude to women: they are trivialised, and then they are blamed. It's time for change.

Catherine Riley is Head of Communications for the Women's Equality Party.