“Big food” is threatening public health and adding to the growing obesity epidemic in the UK, increasing the risk of cancer and other diseases, campaigners said today.

Trends in larger “Americanised” portion sizes, spreading to Britain from the United States, are contributing the nation’s increasing weight problems, the World Cancer Research Fund (WCRF UK) said.

Health experts, speaking at the Keep Big Food Out Of Britain conference in London, said that while people thought they were getting a good deal by opting for “supersize” and “value” meals they were actually putting their health at risk.

Dr Jeffrey Prince, of the WCRF UK, said trends in enormous portions served widely across the US were linked to the increases in weight of the average American.

As a consequence, the country had also seen an increase in chronic diseases such as cancer, heart disease and type 2 diabetes.

“A survey conducted in the US by a WCRF International affiliate shows that Americans continue to eat everything they are served without awareness that the serving sizes are double or triple what they were,” Dr Prince said.

Professor Philip James, chairman of the International Obesity Task Force, said while people were aware of the importance of eating fruit and vegetables, they were less knowledgeable about the need to cut sugar and fat in their diet.

“Obesity increases the risk for many of the most common cancers. Already it is estimated that in the European Union more than 70,000 new cases of cancer are attributable to being overweight each year - and 9,000 of those cases are in the UK.

“Obesity and diet-induced diseases already rival smoking as one of the major causes of preventable death,” Professor James said.

The WCRF UK called on the Government, the food industry and consumers to take action to change the way people eat in a bid to tackle obesity.

Nutritionist Paula Hunt, speaking at the conference, said: “People think they are getting a bargain when they get more food for just a few pence more.

“What isn’t a bargain are the extra calories and fat that comes with these ‘value deals’. But, by exerting a bit more control and deciding for ourselves how much we should eat, we can face bigger portions without fear and stay a healthy weight.”

The conference was taking place at the Royal College of Physicians in central London. The Food and Drink Federation (FDF) said the WCRF had “missed its target”.

Martin Paterson, FDF deputy director general, said: “UK food and drink manufacturers provide many of our favourite products in a wide variety of sizes and styles to suit consumers’ varied nutritional needs and tastes.

“This widens people’s choices for building a healthy, balanced diet. Larger packs are not always consumed by one person or at one session and parents can now often choose mini products and multi packs.

“The industry recognises that it has a part to play to combat obesity and works with the rest of the food chain, Government and educators to help people learn more about food and nutrition.”