A BRIEF missive from inside The Flexibility Trap.

That's the title of a new report from the Timewise Foundation, a social enterprise that supports and lobbies for those who want flexibility in their careers, without writing off their promotion prospects.

It makes depressing reading because, though more than a quarter of Britain's labour force work part-time or flexibly, this research shows that most (77%) feel trapped in their present jobs because quality part-time vacancies are as rare as hen's teeth. Three-quarters of the 1000 part-timers consulted said they haven't had a single promotion since switching to fewer hours. Around 40% had taken jobs well below their skill and (full time equivalent) pay level. And many have been obliged to accept demotion (known in the recruitment business as "backsliding").

Of course, this issue isn't just about fairness and wasted talent. It's also about gender, since most part-time workers are women, usually trying to fit paid work around caring responsibilities. Currently an estimated 600,000 British mothers (including rising numbers of single mothers obliged to look for work by benefit changes) say they feel "locked out" of the workforce. In some cases that's because previously full-time workers are accepting part-time jobs because that's all they can get.

It's easy to see how this combination of backsliding, being trapped in part-time work and being locked out of it altogether is contributing to a widening gender pay gap. Only 3% of part-time jobs advertised pay more than the full time equivalent of £20,000 a year.

That's why Karen Mattison and Emma Stewart got together to form the Timewise Foundation, which has launched an online careers service (womenlikeus.org.uk) with funding from the Cabinet Office.

I should declare an interest in this subject, as I've worked four days a week for several years, following a period of jobsharing, while my children were small. These days people often say things like: "You'd be running that place by now if you'd always worked full time." Maybe, maybe not. I certainly kissed goodbye to any serious promotion prospects by becoming a part-timer.

Why? Because, while some employers (academia, retail banking and the public sector, for instance) are better than others, Britain more than anywhere retains a rigid work culture in which there is a false correlation between commitment and long hours. Conversely, jobsharers and part-timers are still perceived as not doing a "proper job". It still carries a stigma. Just imagine the curl on Alan Sugar's lip if a candidate for The Apprentice requested a four-day week.

Yet, looking back, I'm convinced that my jobsharer and I represented excellent value for money. Together we were more productive and energetic than one stressed-out worker trying to juggle full-time work and childcare could ever have been. Perhaps, grateful to have a decent work-life balance, I curbed my own expectations. Now, as I contemplate my dotage on a dot-sized pension, I hope my daughters don't have to make such a costly sacrifice.

Maybe the Coalition's Children and Families Bill, extending the right to request flexible working to all workers, will help dismantle society's perception of traditional gender stereotyping in the workplace, even if the 26-week qualifying period will continue to act as a barrier to women getting jobs.

Ultimately, wouldn't life in Britain be improved by challenging the polarisation between work-rich time-poor dual income households where parents regularly fall asleep trying to read bedtime stories to their children and families struggling to survive on a patchwork of dead-end part-time jobs? Why shouldn't flexibility be the default option? Why shouldn't most job ads carry a rider to the effect that part-time or flexible working will be considered?

Meanwhile, we need more inspirational examples of those like four-day a week Katie Bickerstaffe, chief executive of Dixons UK and Ireland and Belinda Earl, three-day a week style director at M&S, who feature in the Timewise Power Part-Time Top 50 and who challenge those who still insist that such a combination is an oxymoron.