It comes after it emerged that three-quarters of Scottish councils are offering pupils ham which contains nitrites. Experts have warned that they can lead to an increased risk of bowel cancer.

A total of 23 councils across Scotland serve ham which contains nitrates, according to a Herald on Sunday investigation. And 18 said nitrites were present in other meats such as bacon and pepperoni.

Yesterday, Labour health spokeswoman Monica Lennon said: “There should be no hesitation over nitrites; they must be removed from school menus immediately.

Processed meats with a clear link to cancer should be removed from school menus, opposition have parties have said.

“People will rightly be asking why many of Scotland’s councils are still serving up meats containing these chemicals despite a clear link to cancers and other health issues. We need to know where else nitro-meats are being served? Our hospitals?

“Bowel cancer is a major public health problem in Scotland and we have a higher rate of bowel cancer than most other countries in the Western world. The Scottish Government must take immediate action and work with councils and the wider public to raise awareness of the risks associated with a diet high in processed meats.”

Each 50-gram portion of processed meat eaten daily is believed to increase the risk of bowel cancer by 18 per cent.

School meals are delivered by local councils but have to comply with nutritional standards set nationally.

Edinburgh City Council was among those which confirmed the ham it uses contains nitrates, along with bacon for secondary schools.

East Lothian said it was present in dozens of processed meat products, including ham, pork, corned beef, chipolata links, bacon and gammon steaks. Scottish Borders serves bacon and ham containing nitrites, as well as pepperoni it offers.

Supermarkets, including Waitrose, Aldi, Asda and Tesco are now offering nitrate-free meat, including bacon made without the preservative.

It emerged last week bowel cancer among Scots youngsters is on the rise. There were eleven cases among 15 to 19-year-olds between 1992 and 2011. That trebled to 32 in the five years between 2012 and 2016.

The Scottish Government consulted last year on proposals from a scientific technical working group aimed at making school meals healthier – including a “maximum level” or red and processed meats served in schools.

A spokesman said: “We are carefully analysing responses before finalising revised regulations.”

Local government body Cosla said councils “adhere closely” to national nutritional standards.

Call to ban cancer-causing meats from menus in Scottish schools