A piece or two of dark chocolate might feel like the guilty pleasure that gets you through a long day.

But it may be that there’s nothing guilty about it – even if you treat yourself five times a week.

A study suggests eating dark chocolate can slash the risk of developing type 2 diabetes by more than a fifth.

Researchers from Harvard University in the US tracked the chocolate intake of more than 300,000 healthy people for an average of 25 years each.

They found dark chocolate was associated with a lower risk of disease, while those who ate milk chocolate were more likely to gain weight. And they said eating a few pieces five times a week could be the right amount.

Chocolate contains high levels of flavanols – a compound found in fruits and vegetables – which have been shown to boost heart health and reduce the risk of type 2 diabetes.

The link between chocolate consumption and diabetes risk is ‘controversial’ due to inconsistent results, although the experts note that most previous studies did not explore the difference between types of chocolate. 

These subtypes – dark, milk and white – have varying levels of cocoa, sugar and milk, which ‘may influence the association’, they said.

The study, published in The BMJ, used data from three long-term studies of healthcare workers.

After adjusting for lifestyle and dietary factors, those who ate a 28.3 gram serving at least five times a week were 10 per cent less likely to develop type 2 diabetes compared with those who never or rarely ate chocolate.

Diabetes UK estimates 4.4million Britons are living with a diabetes diagnosis, while a further 1.2million are unaware they have it.

Dr Lucy Chambers, of Diabetes UK, said: ‘Eating more dark chocolate might sound appealing, but, while it can be enjoyed in moderation, we wouldn’t recommend it as a strategy for reducing your risk of type 2 diabetes.’