ASDA has scrapped the four-day working week after staff complained the system was draining them.
The supermarket launched a pilot of flexible working patterns for managers last year – but staff rejected the reduced working hours.
Staff in the trial at 20 Asda stores worked 44 hours over four days instead of five for the same pay.
However, staff said the 11-hour shifts were “physically demanding” and left them on an extra day off.
It comes as Britain’s biggest trade union Unison – one of Labour’s biggest financial backers – prepares to lobby the new government to introduce a four-day week.
Labor advisers downplayed the prospect of legislation for a shortened working week, despite the unions formally backing the idea.
Unison was among the donors who handed Labor large sums of cash in the run-up to the election, with more than £150,000 going to the party.
As well as exhaustion, Asda workers were struggling with early starts and late finishes in a four-day week.
This particularly affected employees who relied on public transport to and from work.
And parents said the longer days made school drop-off and pick-up more difficult.
In response, Asda abandoned the pilot project along with a nine-day fortnightly trial period.
The grocer is still testing a working model that involves 39 hours over five days – which works out to almost eight-hour shifts – without a pay cut.
Asda said it had proved more popular – some workers on the four-day-a-week trial previously asked to switch – and would run until the end of 2024.
A pilot project for a new retail structure that will add additional paid managers in some departments is also continuing.
An Asda spokesman said: “Last year we started trialling four different flexible working models for managers across 20 stores.
“In April we announced that two of these trials, the new retail structure and five shorter working days, would run until the end of the year following positive feedback from colleagues in these stores.
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“We will continue to test different flexible working patterns to assess how they can benefit our colleagues and our business.”
Other companies are also turning against the idea of ​​a four-day work week due to concerns that employees will not be able to cope with longer working days.
Home appliance specialist Domestic & General has ended its four-day-a-week pilot after staff felt “mentally” drained.
D&G chief executive Matthew Crummack told the Daily Telegraph: “Half the team absolutely loved it, half the team didn’t like it at all – it makes the day longer, it’s a bit more intense.”
The company will trial “school hours” to allow parents to work between 9.30am and 2.30pm.
Last year, a London tech firm scrapped plans for a four-day week after realizing it was adding more stress to employees.
Krystal, which provides Internet services, ended the exam as colleagues tried to complete all their tasks in a reduced time.
Krystal then shortened the working day from six hours to five.
The pandemic has seen more employees working from home and adopting more flexible working hours instead of the usual five-day nine-to-five work week.
Some economists argued that fewer hours of work would lower living standards.
And critics say the concept is impossible in customer jobs or 24/7 operations, including those where overtime payments would be an additional cost to employers or taxpayers.