It's Britain's third most popular sandwich — closely behind tuna mayonnaise and cheese, according to polls.

But a shock new analysis may lead ham sandwich lovers to rethink their lunch, due to striking links with deadly type 2 diabetes.

According to the Cambridge University research, which looked at 31 studies involving 2 million people over the course of a decade, eating two slices of ham every day increases the risk of the blood sugar condition by 15 per cent.

It wasn't just ham; eating 100g of any red meat — the equivalent of a small steak — increased diabetes risk by 10 per cent, while links were also found with chicken, albiet weak.

Now, experts have hit back against the paper, published in The Lancet Diabetes and Endocrinology, claiming it 'cannot demonstrate' that red meat causes type 2 diabetes.

Dr Duane Mellor, dietician and spokesperson for the British Dietetic Association, said there was 'missing data' in the studies analysed. 

Although the authors took into account some other factors that could also explain the harm — like BMI, smoking and alcohol intake — they did not control for others that are 'strongly associated' with diabetes risk.

This includes family history, insulin resistance and waist circumference, Dr Mellor says.

'It is possible that the increased risk associated with processed and red meat intake could be a result of these other confounding factors the analysis was not able to take account of.'

Experts say another problem is that the analysis did not differentiate between quality and type of meat.

'Some processed meat is the parma ham and Prosciutto you get from fancy shops, which is likely made with fewer preservatives, compared to the really ultra-processed meat slices in the supermarket,' says Dr Giles Yeo, obesity expert at the University of Cambridge.

He adds: 'The study highlighted a small increased risk of diabetes linked to eating chicken every day — but we don't know how that chicken was cooked.

 'The authors didn't differentiate between fried chicken and home-cooked roast dinners. It may be something in the cooking process that's behind the link, rather than the meat itself.'

The authors are also not clear as to exactly how long you'd have to eat daily ham or steak for to see your diabetes risk jump.

'It's likely to be a few years, perhaps even 10,' says Dr Yeo. 'But very few people eat two slices of ham every day for 10 years.'

Experts have long known of the health risks of processed red meats, like ham and bacon, with several studies linking high intake to bowel cancer and diabetes.

Dr Yeo says the link is thought to be due to a combination of factors, such as the high quantity of saturated fat in the meat products, as well as the presence of compounds called nitrates.

 Nitrates are naturally occuring substances that are added to processed meats in the curing process. 

Studies have found that they can mix with compounds in the gut, sparking reactions that can damage the body's cells.

'The truth is we don't really know for certain why this link exists,' says Dr Yeo.

While he says the study confirms, 'what we already know' about the health risks of processed meat, we musn't 'hyperventilate' about the findings.

'It's important to retain a sense of balance.

'For a lot of people, meat is an important part of their daily nutrition, and processed meat has a longer shelf life.

'Eating most foods in large quanitites comes with risks.'

In October, researchers from Harvard University used data on 200,000 people to show that regularly consuming more than one daily serving of red meat could raise diabetes risk by up to 50 per cent.

However, some of this may be explained by the fact heavy meat eaters were also more likely to be overweight.

 Participants in the study who consumed high amounts of red meat ate more calories overall and were less physically active compared with those who consumed the least red meat. 

'We found that about half of the excess risk with red meat consumption was explained by excess body weight, said study author Dr. Walter Willett of the Harvard T.H. Chan School of Public Health.

'But there was still an increased risk [of developing diabetes] even after taking into account body weight.'