Culture Secretary Tessa Jowell has poured cold water on calls for junk food ads to be banned.
Some experts believe junk food ads are behind rising obesity rates
Her comments came as Health Secretary John Reid launched a debate on how best to improve the nation's health.
Mr Reid wants ideas on how to tackle "worrying trends" in obesity, sexually transmitted infections and cancer.
But Ms Jowell appeared to pre-empt part of the debate, saying she was sceptical about the merits of banning junk food ads.
Over 100 of the UK's leading health and consumer groups have urged the government to ban junk food ads, saying they are fuelling rising rates of obesity.
Figures for 2002 show 8.5% of six-year-olds and 15% of 15-year-olds were obese. Experts predict 40% of the population will be obese within a generation.
The groups include three medical royal colleges, the British Dental Association, the British Heart Foundation and the Consumer's Association.
Who backs a junk food ad ban?
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
Sustain, a group which promotes better food and farming, is leading the campaign. Charlie Powell, its project officer, has insisted that a ban is the only way forward.
"The food industry has proved itself incapable of acting in a socially responsible way," he said.
"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."
But speaking on BBC Radio 4's Today programme, Ms Jowell said she was sceptical about the merits of banning junk food ads.
"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 said.
"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.
The Food and Drink Federation also rejected calls for an ad ban.
"Strict codes of practice already exist," said Martin Paterson, its deputy director general.
Mr Reid's health questions
Should there be rules on where sweets and cigarettes are displayed in shops?
Should the government make contraception more widely available?
Should smoking be banned in public places?
Should there be restrictions on what can and cannot be advertised, such as junk food?
Should the government do more to encourage people to exercise?
Source: Choosing health, Dept of Health
"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."
He added: "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."
The row threatened to overshadow the launch of John Reid's nationwide debate on improving public health.
Public health debate
The health secretary wants to gauge public opinion on a range of possible measures to boost the nation's health outlined in a consultation document called Choosing health.
They include a ban on smoking in public places, better food labelling, restrictions on advertising and encouraging people to walk and cycle more.
The results of the debate will feed into a white paper on public health to be published later this year.
"It's not setting out government policy - it's asking questions in order to
stimulate debate," Mr Reid said.
"We want to see everyone making their voice heard - because everyone's future
is at stake."