Diet programmes such as Weight Watchers, Slimming World and Rosemary Conley are cheaper and far more effective than those run by the NHS, according to new research.
Offering patients counselling in diet and fitness via GP surgeries or pharmacies is "ineffective", as are programmes run by NHS food advisers and dieticians.
Experts said this cash would be better spent on encouraging people to attend classes run by commercial companies. The study, published online in the British Medical Journal (BMJ), involved 740 obese or overweight men and women recruited from one NHS trust in Birmingham.
They were divided into six groups and attended either Weight Watchers, Slimming World, Rosemary Conley, a group-based NHS programme run by advisers and dieticians called Size Down, one-to-one counselling sessions in GP surgeries, or one-to-one counselling in pharmacies.
A comparator group was provided with 12 vouchers for free entrance to a local fitness centre. All the participants were assessed after 12 weeks and again at the one year mark. At 12 weeks, data was available for 658 of the participants and 522 after one year.
All programmes achieved weight loss at 12 weeks - from an average of 1.37kg in the GP group to 4.43kg in the Weight Watchers group. But the NHS programmes were found to be no better than the control group (people exercising at a local fitness centre) at this point.
At the one year mark, all the programmes except the GP and pharmacy groups had resulted in "significant weight loss".
However, Weight Watchers was the only programme to achieve significantly greater weight loss than the control group - and was the best attended group. Compared with the NHS programmes, commercially run ones meant people typically lost an extra 2.3kg.
The authors, from the University of Birmingham, the Nursing and Midwifery Council and NHS South Birmingham, concluded: "Commercially provided weight management services are more effective and cheaper than primary care-based services led by specially trained staff, which are ineffective."
A spokesman for the Department of Health said: "Weight management programmes can be very cost-effective and make losing weight easier for some people, but the best way to lose weight will be different for everyone. The local NHS must think about which weight management service will work best based on an individual patient's needs."