People who work longer hours are more likely to be obese, a study suggests.

Work-related stress and a lack of time for exercise could explain why those who are last to leave the office are more likely to pile on the pounds, researchers say.

Campaigners have jumped on the findings, being presented at the European Congress on Obesity, in Istanbul, to call on ministers to introduce a four-day working week.

But critics described claims that the policy would make Brits slimmer as 'dubious' and warned many could not afford the hit on their income.

The international study by Dr Pradeepa Korale-Gedara, of the University of Queensland, Australia, found countries that worked longer hours typically have higher obesity rates.

She said workplace stress increases the hormone cortisol, causing people to store more fat, while they increasingly work in jobs where they are unable to burn energy.

'I have seen research looking at possible productivity gains but now people are looking at it from a different perspective,' she added.

'When people have a more balanced life, they have a better life.

'They have less stress, they can focus on more nutritious food and engage in more physical activities.'

Dr Korale-Gedara said manual jobs in developed nations are now more mechanised, meaning workers on long hours find it impossible to burn enough fat while relying more on convenience food.

Countries such as the US and Latin American countries, which tend to work longer hours, have higher obesity rates.

This was true despite northern European countries consuming more energy and fat on average than Latin American countries.

A 1 per cent reduction in annual working hours was associated with a 0.16 per cent decrease in obesity rates across 33 OECD countries analysed from 1990 to 2022.

Three in ten Brits are obese and the findings suggest a four-day week - or a 20 per cent reduction in working hours - would be linked to around half a million fewer people being obese.

However researchers warn the study does not prove causation and income levels of different countries may also be another key factor.

It comes amid a growing movement in the UK and other developed nations towards a four-day work week.

James Reeves, campaign manager of the 4 Day Week Foundation, said: 'A four-day week on full pay could slash Britain's obesity levels by giving millions the time they need to ditch bad habits and make healthier choices.

'When we asked staff what they had been doing on their additional day off work, the most common answer by far was 'life admin', including things like food shopping.

'Time for these activities during the week frees up the weekend for other healthy activities too.

'It's essential that local and national Government seriously consider the role that a shorter working week can play in improving the health of our communities.

'The 9 to 5, five-day working week is 100 years old and we're long overdue an update.'

The idea of a four-day working week faced strong opposition from the previous Conservative government but former Labour deputy leader Angela Rayner has expressed some support for the concept.

Dr Christopher Snowdon, head of lifestyle economics at the Institute of Economic Affairs, said: 'The UK is among the ten countries in this study with the lowest working hours and yet has one of the higher obesity rates, so the idea that a four-day week would make Brits slimmer seems dubious.

'For most people, a four-day week would mean a lower income, and we know that people on low incomes are more likely to be obese.

'Even if they weren't, lower incomes would still be a bad thing.'

The four-day week movement cites the historic change a century ago when campaigners led by trade unions gave birth to the two-day weekend.

The previous norm for Britain and other traditionally Christian countries was a six-day working week with time off only on Sundays.

Campaigners argue this shift will become more necessary as artificial intelligence increases productivity but makes some roles redundant.

The move from a six to a five-day week was sparked by similar pressures during the Great Depression in the US.

Office for National Statistics data shows more than 200,000 workers have switched to a four-day week since the Covid-19 pandemic.

A Government spokesperson said: 'This government will not mandate a four-day work week for five days' pay.

'But through the Employment Rights Act, we're making it easier for flexible working requests to be accepted.'