Ultra-processed foods such as fizzy drinks, packaged bread and cake, cereal and processed meats could increase the risk of cancer, according to a new study.

Processed foods make up to 50% of the average person's diet in some developed countries and could be contributing to rising cancer levels, researchers working in Brazil and France found.

Teams from the Sorbonne in Paris and the University of Sao Paulo found that a 10% increase in ultra-processed food intake was associated with a 12% increased risk of overall cancer.

The research also indicated an 11% increase in the risk of breast cancer.

Foods on the list included packaged breads, buns, pizzas and cakes, crisps, industrially-produced desserts, fizzy drinks, fish and chicken nuggets, instant noodles and soups, and frozen ready meals.

The findings, published in the British Medical Journal, were based on a survey of 104,980 healthy French adults with an average age of 43, measuring their intake of 3,300 different food items.

The foods were grouped according to the level of processing and participants were asked to self-declare any cancer diagnosis, which was then confirmed through their medical records.

Researchers also took into consideration other risk factors such as age, gender, levels of education, whether the participant smoked and family history of cancer.

While a small rise in ultra-processed food intake was associated with an increased risk of cancer, there was no significant link between less processed foods - such as canned vegetables, cheese and fresh bread - and cancer.

Consumption of fresh or minimally processed foods including fruits, vegetables, pulses, rice, pasta and fresh meat, was associated with a lower risk of overall cancer.

The authors said: "To our knowledge, this study is the first to investigate and highlight and increase in the risk of overall - and specifically breast - cancer associated with ultra-processed food intake."

The researchers emphasised that at this stage it was just an observational study and no firm conclusions could be drawn about ultra-processed food consumption and risk of cancer.

Ultra-processed foods often have a higher content of total fat, saturated fat, and added sugar and salt along with a lower fibre and vitamin density.

Contaminants, some of which may be carcinogenic, have also been found in heat-treated processed food products, and many contain legal but controversial additives, such as sodium nitrite and titanium oxide.

Some studies have indicated that these additives may be carcinogenic.

The researchers said the study presented several challenges, such as identifying the specific elements in ultra-processed foods that could give you cancer.

Tom Sanders of the King's College London agreed that the term "ultra-processed food" was "difficult to define".

"The definition excludes many home-made or artisanal foods such as bread, cakes, biscuits, butter, meat, cheese, tinned fruit and vegetables, as well as sugar and salt used in domestic food preparation," he said.

"From a nutritional standpoint, this classification seems arbitrary and based on the premise that food produced industrially has a different nutritional and chemical composition from that produced in the home or by artisans. This is not the case."

The head of cancer research at the Irish Cancer Society said the study reinforces existing knowledge about how best to prevent cancer.

Speaking on RTÉ's News at One, Dr Robert O'Connor said the study tells us that we need to be cautious about processed foods because they are high in sugar and fat and cause weight gain.

He said obesity increases the risk for some people of getting cancer and other diseases.

He said eating a pizza or takeaway will not cause cancer, but having a mixed diet and an active lifestyle is the best way to stack the odds in your favourite for not getting cancer.

Dr O'Connor said some people are born with an inherent risk of cancer regardless of their lifestyle.