Unite, which has 60,000 members in the not-for-profit sector, said many members are required to work shifts that incorporate a residential element, often known as ‘sleep-ins'.
If, during sleep-in time, a member is working or at the employer's disposal, meaning they are available to work should an emergency occur or work has to be carried out, this will constitute working time for the purposes of the Working Time Regulations 1998.
Rachael Maskell, national officer, not-for-profit sector, said: "Some employers continue to ignore this, despite the case law that clearly sets out the position.
"Members who are working such shifts are entitled to receive the NMW, currently £5.80 an hour - if the employer is not paying the NMW a claim for unlawful deduction of wages can be brought." She gave an example of a Scottish mental health charity that paid an allowance for sleeping over, but if staff had to wake-up and deal with residents, they were not paid but offered time off in lieu instead.
"This is clearly unacceptable and flouting the law," continued Maskell. "We believe that a number of the more than 170,000 UK charities are copying the poor practices of this Glasgow-based mental health charity."
Unite is also concerned about the amount of rest breaks its members are entitled to - a worker has to have 11 consecutive hours rest in every 24 hours.
To alert its members, the union has published a briefing this month on residential work and urged abuses to be reported to Unite's regional officers.