PICTURE perfect or impossible ideal? As Madonna celebrates her half century with a photoshoot for Vanity Fair the size zero debate gathers ever more momentum.

It seems either celebrities have a bit of the chameleon in them, able to change their shape at will, or fame's best friend, the airbrush, has been busy at work.

Stepping out of the gym next-door to her London home, the pop princess displayed an even more muscular and veiny upper body than usual.

However, posing in only a basque, briefs and knee-high boots for her tenth Vanity Fair cover (released tomorrow), Madonna appears remarkably slender and smooth by comparison.

Actress Kate Winslet, an outspoken advocate for the fuller figured woman, came under fire in 2003 when she appeared on the cover of GQ magazine in a picture that was noticeably enhanced to make her appear slimmer.

And Keira Knightley's bust famously gained about five inches on the US posters for the film King Arthur in 2004.

Amid concerns they are promoting unrealistic body images, magazines are now being urged to ban the use of airbrushed images of models and celebrities in their publications.

The move comes after the Model Health Inquiry, chaired by Baroness Kingsmill, reported in September that digitally enhanced pictures could "perpetuate an unachievable aesthetic".

Now the Periodical Publishers Association trade body (PPA) has appealed to the fashion industry to introduce a voluntary code to govern the use of so- called Photoshopping - digital enhancement giving models unrealistically perfect figures.

The PPA, which represents about 400 UK publishing companies, has invited editors from "glossies" such as Vogue, Hello!, Elle and Grazia to join working groups to discuss best practice regarding the use of digitally enhanced photos.

Kerry Neilson, the PPA's director of legal and public affairs, said: "Using digital technology to adjust images is a technique widely used across all media from promotional material to films to the web.

"As this is a complex issue and there is no pre-determined consensus across the industry, we are currently canvassing views.

"We look forward to developing discussions with both the British Fashion Council as well as editors within the magazine sector."

The move comes in the same week that Professor Janet Treasure of the Institute of Psychiatry at King's College, London, an eating disorder expert, warned that society's obsession with being slim is encouraging dangerous diet-binge cycles and sometimes bulimia.

Ronnie Murray, a former bulemia sufferer who now works as an eating distress coach with adults and young people in Bournemouth, welcomes the move.

"Banning airbrushing of models would be a fantastic step forward in the quest to promote self acceptance in girls and women.

"Those who have an obsession to be thin, who incidentally range from children as young as eight right through to women in their 60s, are hugely influenced by the media culture of magazines,TV and the internet. The message often portrayed by the media is You are how you look' and your appearance is equal to your validation and worth as a person.

"We know that eating and body image distress is increasing with the obesity epidemic. 97 per cent of women are unhappy with aspects of their bodies - indeed it has become normality to feel inadequate physically," Ronnie continues.

"I'd like to see more normalised, realistic and inspiring images of people's lives in the media that celebrate their differences in shape and size. And a move away from promoting the concept that image is everything."