Consumers should be wary of online advice, say experts
Health Which? looked at ten diet sites and found seven fared badly.
Then they used fictitious dieters, with particular health problems, to test out the sites' advice, and claim most did not take their individual needs into account.
Health Which? and diet experts said internet diet sites should be used with care, and people should be careful who is giving out advice.
Health Which? said people should look for a named, qualified professional, a site which personalises advice, and which warns about the risks of losing too much weight or doing too much exercise.
'Jack and Molly'
Health Which? created two characters to test the sites.
Jack was 42, 6ft (1.82m) tall and weighs 20st 12lbs, (133kg) meaning he is classed as very obese.
He was at risk of health problems including heart disease, diabetes and joint trouble, and has not been able to stick to a diet and exercise plan the GP gave him.
Molly, 22, was 5ft 5inches tall (1.65m) and weighs 8st 9lbs (55kg).
Health Which? said tntgetfit.com was the only one to offer reasonable advice, but only as the weeks progressed and Molly and Jack updated the site on their progress.
Ediets.com failed to tell Jack his weight was a health-risk, gave him no good advice on how to improve his diet.
Molly lost a total of 13kg but was not told she had lost too much, and the site even suggested she reduce her calorie intake further.
The site was given five out of 10 for the accuracy of its advice.
Ftbay.net, which Health Which? gave one out of 10, advertised "personalised programmes" but did not provide them, and calculated Jack's body fat wrongly.
The site told Molly to reduce her body fat percentage and said she should aim for 58-59lg - higher than her original weight.
Caution
Emma Copeland, principal researcher on Health Which, said: "We only found one example of relatively good practice out of ten sites we looked at.
Health Which? wouldn't recommend internet diet sites as the best source of information and advice about losing weight safely.
Dr Wendy Doyle, spokeswoman for the British Dietetic Association, told BBC News Online: "People should approach these sites with caution.
"The danger is you don't know who you're dealing with, what qualifications they have to be giving you the advice - and a lot of them seem to be associated with selling things."
She warned people should beware of any site promising huge weight loss over a short period of time, and said people looking for healthy dietary advice should contact their GP, or the dietetic department at their local hospital.