Ms Truss has said the plans could result in job losses and higher food costs for consumers, but Downing Street insists they will go ahead.
She has also voiced fury about related proposals, to cap the calorie content of millions of meals sold by restaurants and supermarkets.
Those plans, revealed by The Daily Telegraph, have met a major backlash from food and drinks manufacturers, and triggered accusations of “nanny state” measures.
The introduction of calorie information on menus was proposed last summer in the Government’s updated childhood obesity strategy. However details of how it will work – including a timescale for the compulsory measures – have yet to be agreed.
Deliveroo said it intends introduce the changes voluntarily, in response to public demand, and a commitment to making healthy eating more accessible. However, the firm is also urging ministers to exempt small firms from compulsory labelling.
It has signed up 500 restaurants from six major restaurant brands – Pho, Yo Sushi and Mexican chain Barburrito, Burger King, KFC and Jamie’s Italian – who will start publishing calorie counts on the delivery portal from next month. And it will write to all UK brands using the platform – encompassing 17,000 restaurants – urging them to sign up for the measures.
The takeaway service is also going to introduce a new menu, picking out dishes which meet specific criteria – such as low in fat, salt and sugar – with meals flagged as “nutritionist recommended” if they pass muster.
The company said it aimed to “dispel the myth” that takeaways could not be healthy.
It follows polling of more than 2,000 adults which found 54 per cent wanted more information about the calorie content of foods ordered for delivery.
Libby Andrews, marketing director at Pho said: “So much of our menu is healthy enough to eat daily, and we’re excited to share this information with our customers.”
“We hope it will make Pho an even more popular option for a healthy night in.”
However, she said the restaurant had no plans to introduce calories on their menus in restaurants.
Jamie’s Italian intends to put calorie labels on its restaurant menus next month, as well as posting them on the Deliveroo portal, while Yo Sushi, Burger King and KFC already display calories in restaurants.
Victoria Robertson, head of food innovation at fast-food chain KFC, said: “It’s really important to us that we give our fans all the right information they need to make informed choices, so we’re very happy to work with Deliveroo to display clear, understandable calorie information on the app.”
Will Shu, founder and chief executive officer of Deliveroo, said: “Deliveroo’s outlook is simple: the way to eat healthy is by having more information and more selection.”
Chris Snowden, from the Institute of Economic Affairs, said compulsory calorie labels could prove devastating for small establishments.
He said: “Calorie labelling is a nice idea in principle but it would be disastrous for pubs, restaurants and cafes in practice.
“Large chains with fixed menus will have no problem complying, but it will be hugely burdensome for smaller establishments. Most pubs, restaurants and cafes change their menus regularly and do not have fixed serving sizes. If this legislation comes in, it could spell the end of the dish of the day. By encouraging the government to impose calorie labelling on the entire out-of-home sector, the large chains are effectively trying to nobble the competition.”
Caroline Cerny, Obesity Health Alliance Lead, said compulsory measure were needed to tackle the UK’s worrying obesity rates, which are the highest in Western Europe.
Two thirds of adults are overweight or obese, as are one in three children leaving primary school.
She said: “This cannot be a piecemeal, voluntary approach – calorie labelling should be mandatory for all restaurants, cafes and take-aways, with no exemptions, to create a level playing field and ensure people are able to make informed choices about the food they eat, wherever they choose to eat.”
Tam Fry, from the National Obesity Forum, said: “For years this sector of the food industry has got away with delivering little more than processed junk and the customers are now calling time. They want to know more about what’s coming to them and adding calorie labels is a good start. The cynic will argue that Deliveroo it is acting before government makes calorie labelling mandatory but that’s immaterial.”
A Department of Health and Social Care spokesperson said: “This shows industry is making positive changes in response to customers demanding better information about the food they eat. “Last year we announced our intention to introduce mandatory calorie labelling on menus to give all families the information they need to make healthier choices when on the go. We’re committed to involving businesses in this, which is why our consultation invited their views on how to make sure the process works for them.”
Additional reporting Tom Mitchell