It was 8pm in the offices of a PR agency in London’s West End and Valerie O’Brien was seething. Her two female colleagues had long since departed in the frantic rush to pick up their children from nurseries. Her male boss had been unavoidably detained with a “client meeting” at a Soho club.

Yet again, O’Brien, single and “child-free”, was left holding the corporate baby. This time it was a last-minute project for a new account.

“It’s not fair,” she raged, resorting to the aggrieved language of the playground. Bang had gone her hopes of going to the play starring Kim Cattrall of Sex and the City.

“What really, really annoys me,” she said last week as she recalled the incident, “is the assumption I will always fill in, that because I don’t have a child I don’t have a life.”

At 41, O’Brien is one of a growing number of people who are remaining single, childless and increasingly feeling nobody is looking out for their interests. She feels particularly hard done by over holidays and having time off.

“It’s one thing for a mother to demand the morning off to take little Samuel to the doctor, but if I ask for time off because I’m expecting a furniture delivery I know what the answer will be,” she wails. “Nobody cares about me!” But her real ire is reserved for parental leave, an issue on which Tony Blair cranked up her resentment even further last week.

Among Labour’s “pledges” for a third term were promises to give mothers an even better deal. Gordon Brown, the chancellor, has talked about extending paid maternity leave to nine months, and Labour has proposed that fathers should be given the option to share the mother’s leave.

“Management rarely provides adequate replacement cover for maternity leave. The net effect is that those of us who don’t have children are expected to pick up the slack,” said O’Brien. “I’d love six months off with pay! Perhaps then I’d find time to meet a husband.

“But I can’t possibly say what I think in the office. People would accuse me of being bitter. I have to express my feelings in secret to other non-mothers. We’re like the freemasons, a secret office club.”

It is a simmering resentment that threatens to divide the workforce across battle lines that transcend gender and class — parents versus non-parents. Inevitably, within those groups, it is the conflict between mothers and non-mothers that is most intense.

“Parents, particularly mothers, are rewarded for putting their families first. That is their priority outside of work,” said Manda Rigby, 42 and childless, who is a company managing director from Windsor.

“My priority outside of work is my friends, followed by Bolton Wanderers. But I can’t very well say I’m leaving early because Bolton are playing at Arsenal. Yet those with families play the ‘kid card’ all the time. Companies will accept coming second to children, but not much else.”

Are such hard-working singletons really martyrs? In the eyes of many parents, struggling to balance work and families, the “child-free” should be grateful not angry. Without children, they say, everyone’s future would be bleak.

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