Predictions on the number of obese and overweight people in the UK may dramatically underestimate the worst-case scenario, says the National Obesity Forum in a new report.

The Forum, led by a GP, wants urgent action to tackle obesity. It warns that the seminal Foresight report of 2007 which said that half the population of England could be obese by 2050 if sufficient action was not taken could have been an underestimation of the scale of the obesity crisis. Public Health England recently estimated that 60% of men, 50% of women and 25% of children could be obese by that date.

The report, the State of the Nation's Waistline, calls for hard-hitting obesity campaigns similar to those intended to stop people smoking. It also wants family doctors to raise the issue with overweight patients and says GPs should be trained and paid to start conversations with their patients about their weight.

"We're now seven years on from the Foresight report. Not only is the obesity situation in the UK not improving, but the doomsday scenario set out in that report might underestimate the true scale of the problem," said David Haslam, the forum's chair.

Health secretary Jeremy Hunt, who recently said Britain was not turning the tide as the US appears to be doing, was correct, said Professor Haslam, but "the time has long passed for national soul searching". "There needs to be concerted action. There is a lot more we can be doing by way of earlier intervention and to encourage members of the public to take sensible steps to help themselves – but this goes hand in hand with government leadership and ensuring responsible food and drink manufacturing and retailing."

The government's social marketing campaign Change4Life is well intentioned, he said, but not enough. "We've seen hard-hitting campaigns against smoking and it's time to back up the work that's already being done with a similar approach for obesity."

GPs were reluctant to start the conversation about weight with their patients, the report says. It calls for GPs to be awarded quality outcomes framework points, which increase their salary, for raising the issue and providing guidance, as well as referring them to other help such as weight management programmes.

But, says the report, "The fact remains that having a conversation with patients about their weight is not an easy job for doctors." The subject was emotive and carried a stigma. "Part of the result of this is that being called fat is viewed as an insult and that no one likes to be told that they are overweight, even by a healthcare professional who has the best interests of the patient's health at heart," it says. GPs could be helped with training to find the right way to discuss weight, so that patients do not get angry or reject their advice.