By Laurie Penny

Across the country last week, thousands of women caught the bus to work only to be informed by a prominent poster that 'Career Women Make Bad Mothers'. The slogan was selected to demonstrate the success of outdoor advertising - in this case, its success was in shaming and offending the millions of women across Britain who work for a living.

The founder of the Beta agency, which designed the advert, justified the choice of slogan on the grounds that “vocalising opinion has always been a great British pastime. We want to ... create a brand which truly democratises debate.” Amidst considerable online support, contributors to the popular parents' forum Mumsnet.com started an unofficial campaign to get the advertisements taken down - an interpretation of 'democratic debate' that failed to please the Beta agency, which immediately launched legal action.

Whatever its intention, the campaign certainly caught a mood. The opinion that women who choose to work outside the home are somehow betraying their 'natural' roles as housebound mothers hardly needs more vocalising; it is a theme that has recurred with alarming frequency in the popular press since the Conservative election effort began in earnest in 2009. However, the right-wing commentariat has failed to realise that the notion of a ‘traditional’ nuclear family – with the man going out to work and his wife staying at home to look after the children – has no real basis in historical fact.

Scholars of the Industrial Revolution describe how separate spheres for men and women emerged between 1780 and 1850 as the workplace became separated from the home and a private, domestic sphere was created for women. In 1737 over 98 per cent of married women in England worked outside the home, and it was only in the early 20th century that the majority began to work as housewives, partly as a result of specific legal sanctions on the part of central government, a process that some historians term ‘the enclosure of women’.

There is little historical precedent for the ‘traditional’ family. The implication that women are naturally inclined to avoid paid work and confine themselves to domestic drudgery and childcare is pure myth, but it suits the prevailing conservative agenda to play into a fantasy that women of previous centuries were exclusively tame, domestic creatures who did no work besides childcare, until the advent of that modern monster, the ‘career woman’.

The attack on ‘career women’ is doubly effective as an attack on female power itself. No cleaner, cook or dinner lady has ever been branded a 'career woman', even though she may work harder and spend fewer hours with her children than her sisters in the media, political and financial sectors. To be a 'career woman', a woman must have had the audacity to become personally powerful outside the domestic sphere – and eight decades after female suffrage, this is still a phenomenon that merits public outcry and private shame. The charge of 'bad motherhood' is intimately and inevitably wounding, as one Mumsnet member, ‘notevenamousie’, described:

"[The ad] on the side of a building today felt like a kick in the stomach. I am being a decent role model and crying blood sweat and tears for my girl. I don't know what else I can do...."

"What about fathers?" asked another working mother on the site. "Are they not raising their child? Are they bad parents for working? No one would ever say 'working men make bad fathers’".

After several hundred women from across the country contacted Beta, their clients and the Advertising Standards Agency to complain, the bus campaign was pulled. Mumsnet received communications from Beta’s Garry Lace and his lawyers, demanding that the firm be "compensated for the hurt, corporate loss and reputational damage that we have suffered as a result of your inability to moderate your medium properly."

Clearly, capitalising on crude, inaccurate social stereotypes that shame women out of economic self-determination is acceptable if you're trying to sell advertising space by encouraging people to 'vocalise opinion' - but if anyone vocalises the opinion that you're a gutless sexist weasel, it's time to sue.

Lace's lawyers went so far as to demand the personal contact details of individual mothers who use the forum. So much for democratising debate. Mumsnet duly 'apologised' for "the strength of feeling expressed by Mumsnetters", saying that site users "assumed that the statement in question was intended to provoke discussion. We now understand that it was ironic casual sexism intended to draw attention to advertising space."

Whoever wrote that advert understood that sexism sells. The resourcefulness of Mumsnet members was not a reaction to one poster alone, but to a prevailing prejudice against working mothers that stems from an advancing tide of sexist political thought. As the recession bites deeper, suspicion of women with the temerity to be employed is fast becoming the misogyny of the moment. Let’s hope that the women of Britain can continue to resist these ugly attacks on our self-worth.