This week's announcement that Ruth Kelly, the Transport Secretary and mother of four,is leaving the Cabinet to spend more time with her family reminded me of my own dilemma: as a parent, how do you know when it's time to pack in your job?

Is it the moment it dawns that you haven't made a single child's sports day, concert or play all year? Or should it be when you're promoted but go home in tears, unable now to see how you can possibly get back for bedtime? Or is it when you're sitting at your desk, in the corner office you always coveted, frantically tapping on your calculator, working out the percentage of time you manage to spend with your children (38.9 per cent, including vacations and Bank Holidays, or a much more comforting 54.2 per cent if you add weekday hours spent together)?

For me, the final straw was sitting on the 6.32am train from St Pancras to Nottingham in 2000 the day school broke up, knowing the wretched deal that I was working on had no prospect of completing until mid-September and I wasn't going to get a single day off in my daughters' eight-week summer holiday.

For more than 13 years I worked in the City, first in high-risk lending and later in venture capital, ending up as a director of a leading private equity business. Although I loved the job, the downsides were the travel, long hours and a brutal corporate culture. A week after returning from a three-month maternity leave, when I was admittedly rather hefty, I lost a deal. My boss's response was: “Ever since you've had that f***ing baby, you've lost your looks and your charm and it's costing me business!” If weddings were just about sacrosanct, not much else was. The all-consuming nature of the work was not conducive to domestic life and I was the only female executive in the office.

It wasn't always like that. When I started in the City in 1987 about 40 per cent of my cohort of graduate trainees were women and it stayed that way until we reached our late twenties and began to marry. Most of my friends came back after their first baby but hardly any after their second. As my family grew - I have three daughters, 10, 12, 15 - I unwittingly became an exotic specimen: a professional working mother in her mid-thirties who was soldiering on. Whenever I stumbled across another, I was fascinated.

The guilt-free and the waverers

Few of us had to work, so what did we have in common? I soon discovered there were two types of City mother, the guilt-free and the waverers. The guilt-free are distinguished by weekend and evening nannies, holidays without children, and evenings out. They don't rush home as soon as they decently can. They even volunteer for conferences and foreign trips. I wished I could be one of them. It looked so much simpler, but I was a waverer, and waverers agonise.

Waverers spend hours boring their partners and friends about whether to walk away from a job they slaved years to get. They're trying to do two things full time, undetected by their kids or boss. Whenever possible I slipped out of the office at about 6.30pm hoping that my colleagues didn't notice. I made ridiculous bargains with myself; getting home before 7pm was fine but five past seven was a disaster. I even tried deluding myself that given their siestas, nursery and then school, my children hardly noticed I wasn't there. Like all waverers I promised I'd stop, but only after the next bonus, baby or deal. But in the end waverers wear themselves down and chuck it in. It's only a matter of time, and for me the time was Nottingham and that early morning train.

I left the station and got into a cab, heading for an industrial estate just north of the city, with only one thought in my head. I couldn't pretend the job was fine for me or my family any longer. Subcontracting a baby's care to a professional nanny was easier than leaving them now that they were getting older. Homework was getting trickier and I wanted to be there to help. I knew few of my children's friends and none of their parents. And then there was music practice and endless after-school activities, all of which I missed. As the taxi drew up outside the low-rise building, my decision was made. I asked the bemused driver to return to the station, I got the next train back to London, rang my boss and told him I wasn't coming back...ever.

So what's it like when you walk away from your fat salary, retire the nanny and become a full-time stay-at-home mum? It would be nice to say that, overnight, my children were more secure, happy and well behaved, but they weren't. They loved their nanny. Claire had looked after my youngest from 12 weeks old. They liked having me around full-time but it wasn't earth-shatteringly great. But even if my children weren't singing “hallelujah” from the rafters, I was...at least to begin with.

For years I'd been living with debilitating guilt and that ended. But there were big downsides. My husband was working in television, so money wasn't an issue, but not earning a salary for the first time since my early twenties was unsettling and, of course, there's the loss of status. I felt this most acutely when asked the dreaded question, “What do you do?” “I used to be a banker” seemed way too defensive, but answering “Nothing” or “I'm a housewife” was dispiriting.

I missed gossip by the water cooler, time with grown-ups, being busy and even deadlines. It wasn't until I left that I realised I counted my year in deals done, budgets and boards. Now there were fewer landmarks and one day merged into another. After I dropped my children at school, empty days loomed ahead and I was bored, and I discovered I wasn't alone because playground mums are also divided into two categories - the content and the waverers.

The content know that they're doing the right thing, for their families and themselves. They don't lament the careers they could have had. They're not troubled by the notion that there's more to them than efficiently ensuring everyone's in the right uniform with a packed lunch. They know life's a compromise and are happy with the decision they've made.

Playground waverers aren't. They miss work but want the benefits of being at home. They can't give up on the idea that there is a way of having it all. It wasn't long before I joined them and found, in the end, that waverers can't resist the lure of work.

I tried teaching, thinking that it would be compatible with children. It isn't, and I didn't have the necessary vocation. I helped out a friend part time and did voluntary work, but it didn't feel like a career. In the end my solution came in a branch of Waterstone's bookshop on Camden High Street, North London. I was looking for a historical novel for my ten-year-old and found that there were hardly any. I'd always wanted to write that kind of book and now I had the time.

Writing's not perfect. It can be lonely and difficult, but I like the discipline and the deadlines. There are still times when I shoo the kids away or boot them into a holiday course because I've got to get things done. But if there's a play, an illness or a school outing, I can be there and, of course, I no longer need to seek reassurance from my calculator.

Caroline Corby is the author of Cleopatra: Escape Down the Nile, and Boudica: The Secrets of the Druids (both Walker Books, £5.99)

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