Children who drink two cans of Coca Cola a week are at risk of developing mental health problems like anxiety and depression, a study has found.

Researchers found that youngsters who consumed as little as 90mg of caffeine a week - the equivalent of one cup of coffee - were more likely to suffer distressing psychological symptoms 18 months later.

Brain scans also revealed subtle differences in brain regions involved in learning and reasoning between children who consumed the highest and lowest amounts of the stimulant. 

From April 2027, it will be illegal for children under the age of 16 to buy energy drinks containing more than 150mg of caffeine per litre. 

Now, the researchers behind the latest study, from King's College London, say the restrictions do not go far enough. 

The study analysed pre-existing data on the drinking habits of more than 9,600 children aged nine to 11. 

This included reports on the number of caffeinated drinks they consumed each week, including coffee, tea and soft drinks.

The data, from the US-based Adolescent Brain Cognitive Development (ABCD) study, also included reports from parents regarding the number of hours their children typically slept each night. 

Researchers also used brain scans to measure the thickness and surface area of the outermost layer of the brain responsible for memory, thinking, and problem-solving. 

The UK scientists then analysed reports on the children's' emotional and behavioural problems six and 18 months later. 

Greater caffeine consumption was linked with smaller surface areas across several regions of the brain, with changes apparent among children consuming as little as one caffeinated drink a week. 

Those who consumed more caffeine tended to sleep for less time, and were more likely to report emotional and behavioural difficulties including signs of anxiety and depression.

Previous research has shown that insufficient sleep can affect everything from concentration and school performance to mental health. 

The findings suggest caffeine from everyday soft drinks may be slipping under parents' radar. 

While energy drinks have been the centre of debate, many popular soft drinks also contain significant amounts of caffeine. 

A can of Diet Coke contains around 46mg of caffeine, while a bottle of Mountain Dew contains about 54mg - not far below the 63mg found in a single espresso.  

'Caffeine is such a normal part of daily life that it's easy to forget young people are consuming it too,'  Professor Divyangana Rakesh, a neuroscientist and study lead author, said. 

Studies have found that two-thirds of adolescents consume caffeine at least once a week, mostly in the form of energy or soft drinks.

Professor Rakesh added: 'We found that even fairly low levels of caffeine were linked to small differences in brain structure and later mental health, and lower sleep duration seems to be an important underlying link.

'And yet, it is completely unregulated.' 

However, the researchers stressed that the findings do not prove caffeine directly caused the changes observed.

Because many soft drinks also contain large amounts of sugar – with a standard can of Coca-Cola containing around nine teaspoons – it is difficult to isolate the effects of caffeine alone.

'Soft drinks also often go hand in hand with less healthy lifestyle habits and more risk-taking behaviours, so we can't yet separate the role of caffeine from everything else that comes with them,' Jiayi Zheng, an author of the study, added. 

'That’s the next question we want to answer, ideally with more precise measures of what young people drink, when they drink it and how well they sleep.' 

Experts not involved in the study have highlighted that the differences found in brain activity were 'very small'.

Dr Charalampos Rallis, an expert in genetics at Queen Mary University of London said: 'The association with later mental health is negligible. 

'The findings were driven largely by soft drinks. However, sugar intake, diet, and broader lifestyle factors may also contribute. 

'The most robust conclusion is consistent with existing evidence: caffeine can reduce sleep in children.' 

As it stands, lower-caffeine soft drinks – such as Coca-Cola, Pepsi, Dr Pepper and Mountain Dew – will not be affected by the proposed ban. 

Separate research has linked high caffeine consumption among teenagers, particularly through energy drinks, to an increased risk of anxiety, depression and suicidal thoughts, with sleep disruption thought to play a key role. 

In response, a spokesperson from the British Soft Drinks Association said: 'The world's leading foo safety authorities have repeatedly confirmed that caffeine is safe when consumed within recommended limits, including by adolescents. 

'This study of children living in the US found associations between caffeine consumption and measures of brain structure and mental health in children, but does not establish that caffeine causes these effects.'