Jeremy Corbyn (Labour), Nicola Sturgeon (Scottish National Party), Sir Vince Cable (Liberal Democrats), Caroline Lucas and Jonathan Bartley (Green Party) signed the letter to Theresa May, prime minister.
They said the measures will save billions for the NHS in the medium term and generate billions more in increased productivity.
The Conservative government has already introduced a tax on high sugar drinks, which came into force this month. Public Health England has also called for a 20% calorie reduction to be achieved through reformulation by 2024.
However, critics argue that these steps do not go far enough to tackle the mounting problem of obesity and related non-communicable diseases.
The opposition letter, coordinated by celebrity chef and health campaigner Jamie Oliver, said the sugary drinks levy was a welcome first step but more action to change the wider food environment is needed to reduce child obesity.
It highlighted further use of taxes to make healthy food cheaper and discourage unhealthy choices.
The UK government is in the process of reforming its Childhood Obesity Plan.
BOGOF ban and 9pm watershed
A child obesity strategy must include an end to ‘BOGOF’ and other multi-buy junk food offers and to junk food marketing on TV before 9pm plus mandatory measures to stop such marketing to children online, said the heads of the opposition political parties.
They also called for a ban on cartoon characters and celebrities used to promote junk food to children and mandatory traffic light front of pack labelling and an end to fake health claims.
In 2017, the Committee on Advertising Practice (CAP) banned ads for food and drink high in fat, salt and sugar (HFSS). However, restrictions only apply to media where children make up over 25% of the audience.
Oliver said the cross party letter means kids’ health is being put above party politics.
“Protecting child health safeguards Britain’s future, which to me is morally above any political position. So this letter is a big moment. These major party leaders have put aside their differences and said, ‘We don’t agree on everything, but we do agree on this’.”
Other measures called for include reformulation to reduce sugar, calories, salt and fat and age restrictions on sale of energy drinks to under 16s.
FDF: Too early for additional regulation
The Food and Drink Federation said in 2016 manufacturers accepted a challenge from the government to reduce sugars and calories over the next few years and industry is pulling its weight.
Tim Rycroft, corporate affairs director at FDF, said it was ‘much too early’ to be considering additional regulation until progress on sugars and calories can be judged.
“We are just one year into the sugars programme (and the calories programme has not yet even begun). But we know this can work because our member companies have already reduced salt in their products by a further 11.4% since 2012, building on more than 15 years of steady reformulation work,” he said.
“It is time for government to resist calls for headline-chasing measures that affect all consumers and instead to invest money behind specific, targeted measures for those people and areas most affected by obesity.”