One in ten have taken drugs to try and achieve their ideal weight.
Over 3,000 women, with an average age of 25, responded to the survey on the newwomanco.uk website.
The survey found women were much more likely than men to criticise other women and put pressure on them to have a "perfect body".
Asked who was most likely to criticise, 85% of respondents said other women, while just 15% said men.
The survey also found that almost 80% of respondents thought about their body size or shape every day, and a third were always on a diet.
Fasting, as a way of losing weight, had been tried by 32%, while 18% had tried de-tox diets, 15% had taken slimming pills and 14% had used laxatives.
One in ten said they had "taken drugs" to try to achieve weight loss.
Women thought thinness equalled success, according to the survey.
Eighty per cent thought slim women were more attractive to men, 60% thought their sex life would be better and 58% thought they had more success in their careers.
Slenderness equals 'ideal'
Over two-thirds of women thought life would improve "considerably" if they were happy with their body.
Seventy per cent felt depressed about their shape or size.
But the vast majority of those who diet put, some, most or all the weight back on again.
One in ten had suffered from anorexia, 9% had suffered bulimia, and 21% admitted they had "binged" on food.
Only 13% of women who had eating disorders had asked for help.
Despite the numbers who were concerned about their physical appearance, 62% did not take any regular exercise, and 64% admitted they did not eat a diet which includes lots of fruit and vegetables.
Women wanted to be thinner, no matter what weight they were - 78% of "normal" weight wanted to be thinner, and most were unhappy with their shape.
Two-thirds of underweight women were unhappy with their bodies and 57% with their size.
Virtually all overweight women were unhappy with their size or shape, and two-thirds thought their size has had a bad effect on their sex life.
Almost all overweight women - 98% - had suffered derogatory remarks about their size.
Plastic surgery
Two-thirds would consider having plastic surgery - 26% said they would "definitely" do so at some point in the future
Half thought bigger breasts boosted a woman's sex life, a third thought they boosted a woman's career, and 78% thought men preferred them.
Ten per cent of those who had surgery said the results were "not good at all", and only a quarter thought there was nothing wrong with having breast implants.
But 94% would be horrified if their partner had a penis enlargement.
'Own worst enemies'
Lorraine Eames, editor of newwoman.co.uk, said: "Women are their own worst enemies when it comes to how they feel about their bodies.
"The truth is, men are happy with us the way we are - it's the women who point out our flaws. It's time we did ourselves a favour and let go of the unattainable "perfect" body dream and celebrated the female form in all its uniqueness"
"There's nothing wrong with women wanting to make the most of themselves, but more and more young women are now turning to plastic surgery in their bid to attain the perfect body."
Adele Wakeham, an eating disorder specialist at the Royal Free Hospital in London, said: "It's completely unrealistic to see actresses and models who are usually seriously to moderately underweight, promoting this image of perfection that just isn't real for many women."
Dr Ann McPherson, an Oxford GP with an interest in women's health, said: "These surveys are incredibly biased because its the women who feel bad about themselves who are going to answer it.
"There's something very wrong here, when women feel so bad about themselves."
She said action should be taken to improve women's self-image, and suggested images in women's magazines should not be looked on as normal.
Dr McPherson also said women should be encouraged and helped to eat and exercise healthily.