In the 19th century the slogan “eight hours’ labour, eight hours’ recreation, eight hours’ rest” became the rallying cry of unions and social justice reformers on both sides of the Atlantic.

More than a century later, that hard-won victory is under threat. In many cases it has already been overturned.

“The 40-hour week is long gone,” says Karyn Twaronite, global diversity and inclusiveness officer at Ernst & Young. Its study of nearly 10,000 full-time managers in eight countries found that working hours have increased sharply since the financial crisis hit in 2007-8.

“It’s not just a US phenomenon,” says Ms Twaronite. “Many other countries are tipping the scales. Since the financial crisis, working more than 40 hours has become the ‘new normal.’”

About half of managers work more than 40 hours, the study found. A third say they are working an additional five or more hours a week.

But the extra hours do not necessarily translate into higher productivity. According to a study by Stanford University, productivity declines sharply after 50 hours, and plummets to almost nothing after 55 hours, so much so that the extra 15 hours is of little value.

John Pencavel, the study author, says this could be because “employees at work for a long time may experience fatigue or stress that not only reduces [their] productivity, but also increases the probability of errors, accidents and sickness that impose costs on the employer.”

Technology is partly to blame for the rise in working hours, combined with the increasingly insecure job market. Although smartphones and remote work software have brought flexibility, too often the need to be “always in touch” is piled on top of — rather than during — the traditional eight-hour day, according to EY’s Ms Twaronite.

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“People are tethered to technology,” she says. “They can’t just go home and switch off. You can be finished for the day but it will be morning in China and you will be expected to respond. Count all those extra minutes online and the hours add up very quickly,” she adds.

Sarah Moore, professor of psychology at the University of Puget Sound, who has studied the decline in working conditions in the US, says that workloads have grown because businesses have downsized so that employees now have to do the work that used to be done by two or more co-workers.

“In addition, especially since the 2008 economic downturn, many workers have reported to us that they are grateful for a job and are unwilling, for example, by turning down overtime hours, to jeopardise their employment.”

Leon Grunberg, professor of sociology at the University of Puget Sound, blames the changes on “more aggressive business targets, more pressure to reduce costs, more technology, and, as a result, more stress,” he says. “Additionally, support functions, for example, human resources, were cut and shifted to managers.”

“As salaried employees who don’t have to clock in, managers are also subject to sometimes subtle and sometimes not so subtle pressure from supervisors and peers to show that they are good, loyal employees by coming early and staying late.”

Nicola Smith, head of economics at the UK’s Trades Union Congress, says the move away from the 40-hour week is part of a long-term shift driven by seven-day-a-week opening for shops, the decline of unions and increasing numbers of women in the workplace.

“It now takes two earners to keep a household going, not just one. We’ve moved away from the traditional breadwinner model,” she says.

Managers are subject to pressure to come in early and stay late

The TUC has coined the phrase “Burnout Britain” to describe the long-hours working culture. Its study of Office for National Statistics data found that the number of people working more than 48 hours per week had risen by 15 per cent since 2010 to 3,417,000.

Ben Willmott, head of public policy at Britain’s Chartered Institute for Personnel and Development, says the shift away from the 40 hour week is “gradual but the trend is definitely in train”.

Even though technology has the power to unshackle workers from their desks, the CIPD says there is an increase in “ presenteeism” — the need to be seen in the office. It found that 30 per cent of employers have seen an increase in people coming in to work while they are sick and 40 per cent a rise in mental health problems in the past 12 months.

“Although the job market has picked up, the outlook is still pretty uncertain, which is one reason why some people are reluctant to take time off sick even when they are ill.” says Mr Wilmott.

“The overall net effect is that many employees find themselves working more hours simply because it is possible and the workplace culture expects it,” says Ms Moore.