Leaders of 70 health organisations have warned Liz Truss against ditching key anti-obesity measures.
In an open letter to the Prime Minister, health professionals say the move would be a "disaster" for the most deprived families and "turbocharge" health inequalities.
Last week it emerged ministers are reviewing the obesity strategy for England - leading to concerns key measures could be scrapped.
These include a ban on TV advertising of junk food before 9pm and the pledge to end buy-one-get-one free deals on unhealthy food and drink.
The open letter - organised by the Obesity Health Alliance - includes groups such as the British Medical Association and the British Heart Foundation.
It calls on Ms Truss to "reconsider any plans to weaken the public health measures" that were supported by previous Tory governments.
The health chiefs stress cutting preventable illness caused by junk food is crucial in tackling the record NHS backlog and meeting targets to cut child obesity.
They add that obesity costs the NHS an estimated £6.1 billion a year and could reach £9.7 billion by the middle of the century.
Dr David Strain, the BMA's board of science chair, said: "It's deeply disappointing to see the new government threaten to throw away the progress we have made tackling obesity without any evidence it would do anything to alleviate the impact of the cost of living crisis.
"Scrapping them would be a disaster for the most deprived families in the UK, families which are most likely to be suffering from obesity both as adults and children.
"Health inequalities are already increasing at a worrying pace and this is just threatening to turbocharge them for no gain."
Chief executive of the British Heart Foundation Dr Charmaine Griffiths said: "If these rumoured reversals to vital obesity policies turn out to be true, then this represents a dangerous step backwards in addressing a major public health crisis".
Katharine Jenner, director of the Obesity Health Alliance, also warned: "If these agreed policies fail to be implemented, we can wave goodbye to the savings of £37 billion for the NHS and £202 billion for wider society through increased productivity that would be generated if we met the 2030 target of halving childhood obesity.
"We strongly urge the Prime Minister to reconsider any plans that would undoubtedly jeopardise these crucial public health measures."
A government spokesperson said: "Addressing obesity remains a priority for the government. Having a fit and healthy population is essential for a thriving economy and we remain committed to doing everything we can to help people live healthier lives."
