Culture Secretary Tessa Jowell says she is sceptical that a ban on junk food adverts would cut obesity rates.
Junk food ads are controversial
She was reacting to a call for a ban from more than 100 of the UK's top health and consumer groups.
Sustain, a group which promotes better food and farming, says 106 bodies back its fight for a new law.
These include three medical royal colleges, the British Dental Association, the British Heart Foundation and consumer councils.
The organisations believe the ban is necessary if the UK is to tackle rising rates of obesity.
But Ms Jowell said a lack of exercise is the primary cause of growing childhood obesity rates in the UK.
"The reason I am sceptical is that we have got to come back to the evidence. We are getting fatter because we are less active," she told BBC Radio 4's Today programme.
"Of course advertising has an impact, but what we have to judge in government...is whether this action would be proportionate."
She challenged advertisers to use their creative genius to promote healthy eating and exercise instead.
One in four men and one in five women in the UK are classed as obese.
Experts believe 40% of the population will be obese in a generation, fuelled by rising rates among children.
Figures for 2002 showed 8.5% of six-year-olds and 15% of 15-year-olds were obese. Many experts believe junk food ads are playing a role.
Last year, Ms Jowell asked the new television regulator Ofcom to review its advertising code as part of efforts to tackle obesity. The Food Standards Agency has also been looking into the issue.
British Dental Association
British Dietetic Association
British Heart Foundation
Child Poverty Action Group
Royal College of General Practitioners
Royal College of Physicians
Royal College of Surgeons
There is growing speculation the government will urge the food industry to sign up to a voluntary code to stop targeting children rather than opt for a ban.
However, Sustain has sent a report to ministers urging them to introduce a law banning junk food ads.
"The food industry has proved itself incapable of acting in a socially responsible way," said Charlie Powell, project officer at Sustain.
"Huge profits are at stake, so we don't believe that they will voluntarily stop promoting junk foods to kids.
"For the sake of children's health, statutory controls are urgently required."
The Food and Drink Federation said there was no need for a ban.
"Food and drink manufacturers take a very responsible view of advertising, particularly of products aimed at children," said Martin Paterson, its deputy director general.
"Strict codes of practice already exist, and these state that ads should not encourage children to eat or drink frequently throughout the day, condone excessive consumption, or suggest that confectionery or snacks should replace balanced meals.
"Obesity is a serious problem and the UK food industry wants to play its part in finding realistic solutions.
"There are no quick fixes. Any action needs to be based on sound science, and requires government and all stakeholders to work together with a commitment to achieving real results over the long term."