A researcher who embedded himself in several London branches of one of the UK's largest supermarkets found that management used a combination of 'flexed-time' contracts and overtime to control worker shifts to meet times of anticipated demand, while ensuring costs are kept to a minimum.

Workers at the supermarket chain were frequently expected to extend or change shifts with little or no notice, often to the detriment of their home and family lives – causing the majority of workers interviewed to feel negatively about their jobs.

Low wages and lack of guaranteed hours, combined with convoluted contractual terms, weak union presence, and pressure from managers that at times bordered on coercion ("...there are plenty of people out there who need jobs") meant that many felt they had no choice but to work when ordered, despite the impact on childcare, work-life balance and, in some cases, health - both physical and mental.

Dr Alex Wood, who conducted the research while at Cambridge's Department of Sociology, has chosen not to name the retailer in the new study, published today in the journal Human Relations. Having spoken with union representatives from across the retail sector, however, Wood believes the practises he encountered are now endemic across major supermarkets in the UK.

The Government's website describes flexible working as something that "suits an employee's needs". However, Wood says there is a critical distinction – one overlooked by the Department of Work and Pensions (DWP) – between workers controlling their own schedules, and management imposing control.

"Control over flexible working enables a better work-life balance. However, such control is the privilege of high-end workers. When low-paid, vulnerable workers experience flexible working time, it is at the whim of managers who alter schedules in order to maximise profits, with little consideration for the work-life balance of employees," said Wood.

The practice of low core-hour contracts that can be 'flexed up' are most notoriously embodied in zero-hour contracts – recently reported to affect over 800,000 British workers. Last year, then DWP Minister Iain Duncan Smith held up a survey claiming to show "most" workers on such contracts find them to be beneficial.

Wood says this is an example of conflating low-end, hourly-paid workers who have schedules dictated by management - those in supermarkets, for example - with highly paid professionals such as consultants who control their own hours of work. While all are technically on zero-hours contracts, their experiences of work are dramatically different.

"It is misleading to claim that flexibility provided by zero-hour contracts is beneficial for 'most' workers' work-life balance, and it is simply implausible to suggest this is the case for low-paid, vulnerable workers who by definition lack the power to control their working time," said Wood, who contributed evidence to the coalition government's zero-hours policy review in 2014.

For the study, Wood conducted interviews with a number of workers from across four of the UK retailer's stores, ranging from check-out operators to online delivery drivers, as well as interviewing union reps and officials. He also conducted two months of "participatory observation": working as a shelf stacker in one of the larger supermarket stores.

His findings have led Wood to conclude that the problem of precarious contracts goes far beyond just zero-hours, encompassing most management-controlled flexible contracts.

At the time of the research, the UK retailer had a policy of new stores reserving 20% of all payroll costs for short-term changes in shifts, which requires around 45% of all staff to be on flexible contracts, says Wood, although interviews with union representatives indicated this was likely higher.

While contracted for as little as 7.5 core hours, all flexible workers had to provide 48 hours of availability per week at the point of application – with greater availability increasing the chances of being hired.

Officially, 'flexed' hours were not to exceed 60% of workers' core hours. However, despite being contracted for a weekly average of just nine core hours, Wood found that standard flexible workers were working an average of 36 hour weeks.

Management used combinations of 'overtime' – additional hours that are voluntary but can be offered on-the-spot – with 'flexed time' – additional hours that are compulsory but require 24 hours' notice – to ensure staffing levels could be manipulated at short notice to meet expected demand.

Both overtime and flexed time were paid at standard rates, keeping payroll costs down, and Wood found distinctions between the two were frequently blurred - disregarding what little contractual protection existed.

"In reality, the nature of low pay and low hours contracts means these workers can't afford to turn down hours," said Wood.

"Whether zero core hours, or seven, or nine - none provide enough to live on. This precarious situation of not having enough hours to make ends meet is heightened by a perception that refusal to work additional hours meant they would not be offered them again in future, something most workers simply couldn't afford."

The stress caused by management-controlled flexed time of low hour contracts, and the impact on home and family lives, were frequently raised by the workers that Wood spoke to.

One worker provided what Wood describes as a "characteristic experience". Sara co-habited with her partner Paul, also employed at the UK retailer. "[W]e've set aside Saturday as a day to do something – me, Paul and my son – as a family... She [Sara's manager] now wants me to work Saturdays... it's all up in the air."

Colin, another worker, described the impact of dramatic schedule alterations to his wellbeing: "I had to change hours, or accept another position, or try another store... I felt really sick, it just hit me, it hit all of us..."

Asim, a union rep, made it clear that management bullying occurred: "People have been told, wrongly, that they can be sacked for it if they don't change their hours."

Under Duncan-Smith, the UK government legislated to ban 'exclusive' zero-hours contracts – those that have no guaranteed hours but restrict workers from getting another job – but Wood says this is simply a straw man, and new DWP Minister Stephen Crabb must go much further.

'It's imperative that Stephen Crabb breaks from his predecessor and recognises the damage which wider manager-controlled flexible scheduling practices, including all zero hours contracts, do to work-life balance," Wood said.

"Policies are needed which strengthen low-end workers' voice. When alterations to schedules are made solely by managers and driven by cost containment, flexibility is only beneficial for the employer not the employees."