For the past four years, desperate television producers have had nothing but sex on the brain -- Sex and the City, that is. The raunchy gal pal comedy wound up in the summer of 2004 after spending six years literally and figuratively penetrating popular culture, influencing fashion trends, and midwifing the birth of a particular brand of aspirational lifestyle.

With the long-awaited (and much-delayed) Sex and the City: The Movie due to hit the big screen in May, two new TV dramas have sought to replicate the trailblazing and phenomenally successful template set by Carrie Bradshaw and the girls -- but with very poor results.

Cashmere Mafia (which has been bought by RTE to air later in the year) and Lipstick Jungle (snapped up by digital channel Living) owe more than a passing resemblance to the multi-award winning show. The executive producer of Cashmere is Darren Star, the co-creator of Sex, while Lipstick Jungle is based on a well-received 2005 novel by the original Sex and the City author, Candace Bushnell.

The similarities don't end there. Both shows focus on a group of rich, successful and impossibly glamorous New York girlfriends as they try to balance their high-flying careers with romance, marriage and kids.

By stuffing these shows with female-empowerment credentials, seductive shots of Manhattan and budget-swelling arrays of fabulous clothes and designer shoes, TV execs believed they had finally struck on the winning formula after years of post-Sex duds (such as Men in Trees and Leap of Faith).

But the critics -- and the viewers -- were not so convinced when the two shows began in the US in January. Cashmere Mafia, which stars Lucy Liu and Frances O'Connor, was called an "entertaining, if pale, sequel to its HBO prototype" by the New York Times, while USA Today slammed it as "snide, condescending and unpleasant".

Meanwhile, Lipstick Jungle, headed by Brooke Shields and 24 star Kim Raver, received equally divisive reviews, ranging from "has some good things going for it" (Wall Street Journal) to "awful" (Entertainment Weekly).

So what went wrong? After all, the timing seemed right. The shows' premieres were deliberately held back until the New Year to fill the ratings void caused by the writers' strike (both series averaged a very middling five to seven million viewers in their first few weeks).

In addition, right now is considered something of a golden age for female characters on television. There is a raft of quality weekly dramas with meaty, complex roles for women at their core, such as Weeds with Mary Louise Parker, Damages starring Glenn Close, and Kyra Sedgwick's The Closer. But why have these two latest glossy shows failed to match Sex and the City in terms of wit, style and zeitgeist-surfing urgency?

Janet McCabe and Kim Akass, research fellows at Manchester Metropolitan University, who have co-edited critical studies of Sex and the City, as well as Desperate Housewives and The L Word, argue that the risqué ethos of Sex has become commodified by rival networks, without any originality or edge.

Dr McCabe explains: "Cashmere and Lipstick look and sound like Sex, but they are just formulaic -- a group of attractive women, financially independent, living in New York. There the resemblance ends.

'While Sex was funny, sassy and compelling, these two are just too knowing and cynical. Sex, for example, asked some very pertinent questions and yet the answers offered by these two shows are not very satisfactory. There is not even an interesting twist on the Sex narrative. They do not seem to be exploring the same kind of issues in terms of class or race or anything; they're just replicating a 'winning formula'."

Akass adds that Sex also benefited from airing on the subscription cable channel HBO, the stable of other envelope-pushing shows like The Sopranos and Six Feet Under, while Lipstick and Cashmere are constrained by the prudish moral codes engrained in American network television.

"Being on HBO meant Sex could say and do things that cannot be said on the mainstream networks," Akass says.

"With this freedom came nuance and innovation. It's almost as if this has been exhausted by the time it gets to the networks. Controversy gets translated into formula."

However, being able to show graphic sex and use bad language were not the only components underpinning the allure of Sex and the City. Ultimately the show was romantic to its core, despite the revolving cast of male (and sometimes female) lovers and all the cheeky talk about sex toys.

"Sex and the City always acknowledged and took pleasure in the romance that we all invest in, such as fairytales of Mr Right and finding romance in New York," says Akass. "Lipstick Jungle and Cashmere Mafia just seem rather cynical takes on the impossibility of that. Sex never lost its belief in the fairytale, even though it critiqued it at the same time."

But perhaps the true key to Sex and the City's popular appeal was best summed up by Jenny Bicks, one of the show's former head-writers, in a recent interview with Entertainment Weekly. "Sex and the City gave voice to a group of people -- single women in their 30s," Bicks stated. "The question is: who is the next group who hasn't had a voice yet? I don't think it's anyone sipping a cosmopolitan in a bar."

- Lisa Jewell