GPs have no incentive to do more than "tick boxes" when it comes to tackling obesity, campaigners said.
The National Obesity Forum said that doctors are rewarded financially for recording the number of obese patients - yet not for doing anything about it.
The organisation will tell MPs that there needs to be an overhaul of the way GPs approach obesity.
At a reception, National Obesity Forum clinical director Dr Matt Capehorn is expected call for overweight patients to be guided towards weight-management programmes.
He is expected to say: "Encouraging GPs to engage with weight management services may be difficult given other pressures on their time.
"Merely drawing a register will not prevent a single overweight person from developing type-2 diabetes or a single obese person from having a heart attack.
"We recognise the obesity epidemic is not easy - tackling it needs action across government departments."