Mumsnet has expanded into an 850,000-strong organisation and is now set to wield real clout in the election campaign

It's been a bumpy but fulfilling ride, its co-founder tells Anne McElvoy

• Gordon Brown (pictured with his wife, Sarah, and

Justine Roberts, centre left, and Carrie Longton) and David Cameron have both taken part in Mumsnet online chats, showing that the community has an influential constituency politicians are keen to court Picture: PA

I APPROACH Mumsnet Towers with caution, after recent coverage has suggested that the sensitive should tread carefully. "An entire tribe of grumpy, itchy, scratchy women," writes one male critic. Another female journalist queries whether the mums' website hasn't become too bitchy for its own good. An online argument about bad language on the site ended up with one user instructing another: "Off you f**k."

Still, regular users have grown to more than 850,000 and Mumsnet is now ten years old (they held a glitzy birthday party last week). Were she a real mummy, she'd treat herself to a double chardonnay from the back of the fridge and an extra order of something unwisely crimson from Boden.

There's an intriguing cultural clash at the heart of the site between the nice mores of middle-class motherhood and sometimes a tone of nettled resentment, which boils over into fiercely crossed swords.

When I meet Justine Roberts, the co-founder of Mumsnet, she's in an airy north London base occupied by around 25 (flexible working) staff with the kind of busy but relaxed air and modern furniture that gives you an attack of office envy. As we might expect, there are several kinds of health-giving teas on offer – and homemade biscuits, naturally.

We're approaching the "Mumsnet election", and the site's premium female demographic hasn't escaped politicians courting the female vote. Sarah Brown hosted a cyber "big night-in", David Cameron has done two web chats and Liberal Democrat leader Nick Clegg has declared himself ready to do an election husting just for the mums online, if the other two play ball.

It's quite a journey for a business run by Roberts and her partner, Carrie Longton, "from our back bedrooms" when her twins were one and she felt the isolation of the intelligent mummy. "It was that time when everyone had an internet idea," she says, "so I thought, 'Why not have a go?'"

Roberts is a City analyst turned football and cricket writer, and Mumsnet was also her attempt to find a flexible job that would be rewarding, but would leave her free to see her children and "not have to squirm if I wanted to pop into a school event".

She's married to the senior Guardian editor, Ian Katz, and now has a family of four – one of whom pops into the office while I'm there and does her homework. Other women must be thinking: "God, I wish I'd thought of something like that."

Yet, she insists, it's been a financially hard road. "We didn't make a penny for the first five years, so you might well say, 'What sort of business plan is that?'" She now makes "a reasonable salary" from her pet project, and its advertising profile has grown massively.

But it's not a big moneyspinner, she says, "until Rupert Murdoch sorts out the internet and we work out what to do about too much being free".

Mumsnet chugged along for years with sensible forums on how to conceive, stop colic once you've had the longed-for baby and generally be a middle-class parent without going mad.

Fishing around, I find tips on getting into top state schools (Mumsnetters are very pro-state school, though there are little corners of private-school chatter, and a whole section on how anxious parents are feeling as the acceptance and rejection letters for secondaries go out this week).

It is to modern motherhood what the old Reader's Digest DIY book was to every household: the place with answers no-one else seems to know. But recently, it's had a choppier reception in the Sunday Times.

Writer Isabel Oakeshott berated some of the contributors for their general assumption that husbands and partners are a barrier to a happy life, rather than part of one. There's certainly some of the crabbier end of feminism on offer. "You have to stand up for yourself," runs one thread, "he may be the Neanderthal." Another (male) writer describes it straight up as "enemy territory – the 'there is no evidence of men here, not even sensitive, understanding, co-operative ones'."

So what does this softly spoken bermum make of the charges? "Well, on the one hand people tease the Mumsnetters as all Boden and cupcake-baking," she says evenly. "The next minute, they're attacked as the harridan classes." She defends the more robust tone of the site compared with other parenting forums. "We do make it clear that if you ask for opinions, people will give them to you and they may not always be what you might like to hear. The community," she says, "determines the content of the site and its direction."

It's a phrase she uses several times and has for me, at least, an unnerving Salem-like undertone. Internet "communities" can be harsh things to be on the other end of. Social networks do tend to take on prejudices and fashions and magnify them – not always kindly. Doesn't she regret that the site she set up to be a supportive haven for bright mums negotiating the rockface of parenthood should end up causing quite so much bad feeling?

"You have to remember that 75 per cent of our users have degrees or beyond," Roberts says. "There are experts on just about everything on Mumsnet. And, yes, there's a kind of insistence on some things: they will correct your grammar, for instance, and they don't like text-speak: people write properly. It's not something we've ever said, let alone imposed. It's just what the community wants."

She thinks a lot of other parenting sites are "basically about validation – you don't necessarily get that on Mumsnet. If you send a mail about your kids getting overweight you're very likely to get messages back saying, 'You're feeding him too many crisps.' But it is important to see its strength as a site where people can trade advice and strong opinions. Our philosophy is basically supportive."

She does acknowledge that the size of Mumsnet these days does "magnify" negative reactions. "You can get 200 replies disagreeing with what you've said. We just remind people that it's not a group of people out to get you."

Mumsnet looks, on the face of it, like a witty and relaxed place to browse, with strings of queries, such as "Am I being unreasonable?" (answer usually yes): but a lot of the replies are very sharply worded and there's now a warning against "personal attacks" at the top of one of the more contentious comment threads.

Big-name guests on the webchats are often treated as big game by the female laptop avengers.

Gordon Brown fell foul of the community for supposedly failing to specify his biscuit choice. Roberts later said that, because of the way they posed the questions, "he didn't avoid the biscuit question because it didn't cross his path".

David Cameron was net-slapped for taking too long to reply online to a query about nappy allowances for children with cerebral palsy. A technical glitch, it turned out, but long enough for one Mumsnetter to mail, "Can't he get a butler to do it for him?"

Environment Secretary Ed Miliband was put in his place about the ecological arguments over disposable nappies.

Roberts now says she's encouraging users to be "polite" to webchatters. "It is like having a guest," she admonishes. Foreshadowing the benign treatment of Labour in the polls, users were more annoyed with the charity manager who broke a confidence about a member of the Downing Street staff claiming they were bullied than they were with the alleged bully, Mr Brown. Is there a block vote, I wonder? "Certainly not. Trying to define Mumsnet politically would be like trying to pin down an octopus with PMT," retorts Roberts, though later she says a lot of regulars are Blairites who are now sniffing curiously at Project Cameron.

When I ask her what are the most polarising issues haunting modern mamas, she immediately cites the working-mother/stay-at-home divide, which looks set to run and run. "Both sides can be over-sensitive and view anodyne comments as criticism."

Some of the more fundamentalist fans don't like the fact that Mumsnet has developed ties with Google. Roberts doesn't see a problem – and I doubt that it will be her last flirtation with the masters of the internet universe. She's just signed a deal with Vodafone to develop a mumsnet app for the iPhone.

It's all a long way from the beginning among the nappy buckets. Mock the mumsnetters at your peril. They have a way of coming out on top.

• Mumsnet Guide is available from www.bloomsbury.com/mumsnetbooks

MUM'S THE WORD

MUMSNET LIKES

Biscuits. Boden. Its own abbreviations, like SWMNBN (she who must not be named). Tips on school admissions. Other mumsnetters.

MUMSNET HATES

Gina You-Know-Who. Useless husbands. Un-green nappies. Patronising bosses. Evasive politicians. Grammatical lapses.

BEST LINES ON MUMSNET

"Hello Baby… Goodbye Pelvic Floor."

MUMSPEAK:

SAHM: Stay at Home Mother; WOHM: Working Out Of Home Mother; DS/DD: Darling Son, Darling Daughter; YA(N)BU: You Are (Not) Being Unreasonable

THINGS TO SAY AT MUMSNET

"Gosh, you're doing so well juggling it all: I don't know how you do it."

THINGS NOT TO SAY

"Can anyone tell me the appeal of a bunch of women stating the obvious?"