It was 11am on Friday when The Sunday Times phoned Mark Mullen, chief executive of Atom Bank. “Hello?” he answered, sounding flustered, a loud cacophony in the background. “Sorry, I’m just in the supermarket, I can’t hear you.” Had he popped in to buy a notepad on the way to an important meeting?

Far from it. Three years ago, on his orders, Atom Bank’s 550 staff went from five days a week to four. So, having worked his Monday to Thursday, Mullen was doing the grocery shop.

The change, he said, has been a success. “We’re profitable, still growing — but we do it with a four-day week.”

As part of the new government’s working rights package, employees in the UK may soon all be given the right to work the Atom way. Under a plan due to go out for consultation in October, the Department for Business & Trade will propose a system of “compressed hours”, where full-time staff can squeeze their contracted hours into four days. Employers would be legally obliged to allow it.

The idea is to give employees flexibility. That could involve doing fewer days, or working different hours — possibly involving evening work after putting children to bed.

Britain isn’t alone in looking at such a move. Belgium passed similar legislation in 2022, and trials have been running in several other European countries. But how practical is it? And what impact will it have on companies and the economy?

A wide-ranging trial last year — in which 61 companies went to four days — suggested it was good for employees, who had lower levels of stress and burnout and took fewer sick days.

However, Mullen stressed that while it worked for a business like Atom, it would not be right for every organisation. “We’re kind of self-service, digitally enabled, so we don’t have a physical distribution service to maintain. That made it easier than if we were running something like a restaurant.”

And as for the productivity of his workforce? Atom cut the total contracted hours from 37 to 34 a week, meaning staff worked slightly longer on those four days. He kept wages as they were. “It had no impact on [total] productivity at all.”

On a per-hour-worked basis, though, Atom staff did become more productive.

Professor Bart van Ark, managing director of the Productivity Institute at Manchester University, said there was not always the same positive outcome when employees were asked to work longer hours over fewer days, with a risk of fatigue and concentration lapses. “After seven or eight working hours a day, your productivity is very likely to decline,” he said.

Richard Fox, a partner at Keystone Law specialising in employment issues, agreed: “It’s not quite as simple as saying a four-day week with compressed hours is less stressful and adds to work-life balance … If you work ten hours four days a week, you can find yourself being exhausted. For some people, it’s better to work more steady hours.”

“I’m definitely not saying it’s right for everyone,” Mullen added. “Some people like a five-day week.”

On the other hand, though, he pointed out that once employees have made the switch to four working days and rearranged their lives around that schedule, they can find it hard to shift back up again to five, perhaps making them unwilling to move to new jobs — good for Atom, but perhaps not for rivals trying to hire new staff.

After the four-days trial last year, 92 per cent of the participants decided to make it permanent. However, some struggled. Mark Roderick, managing director of Gloucester-based engineering firm Allcap, said that the new rosters had made it harder to ensure staff were always on hand to deal with customers all week.

He had trialled compressing working hours down to nine days every two weeks, but even then employees became exhausted after “nine extreme days” rather than ten manageable ones. Allcap abandoned the trial.

It wasn’t just smaller companies that found it hard; Asda abandoned a pilot scheme for managers last year in 20 stores. The employees had compressed their contracted 44 hours into four days rather than five, but they found the 11-hour shifts were too tiring and left them exhausted on their extra day off.

Abigail Marks, professor of the future of work at Newcastle University Business School, warned that the four-day week idea was divisive. “It might work for the office workers, but for the most vulnerable — those working for platforms like Deliveroo drivers who get paid by the job — it does nothing.”

Meanwhile, she warned, people put on four day weeks often struggled to get their tasks done. “Many people on four day weeks tell me: ‘I have to work on my day off, but don’t tell my boss or my colleagues.’”

While there are companies like Atom that have made the switch permanent, it is still rare for entire countries to do so. Belgium’s new rules, which give workers the right to demand to compress their five-day weeks down to four longer days, came into force last November.

It is too soon to draw many conclusions, but early indications suggest there has been no rush to take up the option: an April survey found only 0.8 per cent of full-time workers had switched.

That could be because, at an average of 34.9 hours, full-timers in Belgium already have one of the shortest working weeks in Europe, according to data from Eurostat, the EU’s statistics office.

While there are exceptions, the countries in Europe working the fewest hours tend to be the most productive. In the Netherlands — where workers clock in at 32.2 hours a week, the lowest total — a four-day week has not been enforced, although part-time working is particularly popular among the Dutch compared with other countries.

Despite this apparently relaxed attitude, the Netherlands’ annual output per head, at €53,000 (£45,000), is higher than most peers, including Britain, where gross domestic product (GDP) per head was about £10,000 lower last year.

Having a small population and a services-led economy helps, but companies in the Netherlands tend to invest more in their businesses than Britain and other less productive countries, improving their efficiency as each worker produces more.

The end result is that the Dutch have plenty of time for “niksen” — the wellness trend that translates as “doing nothing.”

Of the big five European economies — Germany, the UK, France, Italy and Spain — Germany has by far the lowest average working hours, at just 34 a week. Employees in Germany have a culture of working hard, with little chatting, but rarely putting in more than a seven or eight-hour day.

GDP per capita is expected this year to come in just shy of €39,000, though that is dragged down by the less advanced former east German regions. Areas such as Hamburg are about twice that figure — a level of productivity that the Germany Trade & Invest organisation largely attributes to high automation in the country. With about 415 industrial robots per 10,000 manufacturing sector employees, Germany lags only South Korea and Singapore in the “robot density” stakes, says the International Federation of Robotics. Britain is the only one of Europe’s big five not to even feature in the top 20.

While Britain considers going down to four days a week, Greece has just gone up to six.

Since July 1, the country’s pro-business government under prime minister, Kyriakos Mitsotakis, has permitted companies in certain industries to ask staff to work a six-day week in return for 40 per cent additional hourly pay for the extra time. The rule, which applies only to private businesses providing round-the-clock services, is not mandatory.

The country’s food and tourism sectors were not included, as they were already allowed to add the extra day.

Unions have complained that Greece is heading in the opposite direction to the rest of Europe, but — faced with a mass emigration of workers since the Greek debt crisis in 2010 — Mitsotakis said he had little choice. He described the shrinking population and shortage of skilled workers as a “ticking timebomb”.

Greeks were already working the longest hours of the main European states, with full-timers clocking up an average week of 39.8 hours. That reflects the low average pay in the country, which, at €17,707 a year, lags all the western European nations except Portugal’s €16,943.

For companies that decide to go from five days to four, the move requires careful planning. Alice Lang, 30, at the Chester marketing agency Marketing Signals, said her firm split the workforce into two teams who alternate between working Monday to Thursday and Tuesday to Friday. “This ensures every employee has a four-day work week each week, but with alternating two-day and four-day weekends.”

Before the switch, brought in last March, she worked seven hours a day; she now does eight. “My work-life balance has improved drastically. When I worked five days, I worked all day, packed workouts, cooking and some socialising into my evenings, and would be completely knackered by Friday.”

Now, she said: “I’m less stressed and, generally, happier, which naturally impacts the quality of my work. I find I can concentrate more, have fewer dips in energy, and just feel more productive. I believe I get the same amount of work done in four days as I used to in five, and my long weekends mean I can go on lots of city breaks. I’d say it’s increased my loyalty to the company, too.”