Overweight people referred by their doctors to Weight Watchers lose twice as much fat as those who receive standard dieting and exercise advice from their GP.

A study funded by the Medical Research Council – the results of which were first revealed a year ago in the Guardian – will encourage GPs to send overweight patients to the commercial organisation. Few other strategies work as well with severely overweight people, who risk heart disease, stroke and diabetes.

The study published online in the Lancet medical journal compared people sent by their doctor to Weight Watchers in three countries – the UK, Australia and Germany – with those who just received standard advice and care. The results in all three countries were similar.

The trial involved 772 overweight and obese adults who were randomly assigned a 12-month Weight Watchers course or standard care from their doctor. At the end of the year, those enrolled in the commercial programme lost an average of 5.1kg, compared with 2.2kg for the rest.

Not everybody completed the course. The weight loss among those who did was 6.7kg at Weight Watchers and 3.3kg for the rest. Those enrolled in the commercial programme were more than three times as likely to lose a substantial 5% of their bodyweight, compared with the others.

"The greater weight loss in [those] assigned to the commercial programme was accompanied by greater reductions in waist circumference and fat mass than in [those] assigned standard care, which would be expected to lead to a reduction in the risk for type 2 diabetes and cardiovascular disease," said author Dr Susan Jebb from the MRC Human Nutrition Research unit in Cambridge.

In a linked comment, Dr Kate Jolly and Dr Paul Aveyard from University of Birmingham say: "The low cost of these programmes (at present about £50–60 for 12 weeks) makes the case for incorporation intuitively appealing."