A legal action over payments for "sleep-in" shifts, which could cost the UK care industry billions if judges rule in favour of workers, has reached the Court of Appeal.
Leading judges in London will consider two recent conflicting rulings on whether care workers who sleep at their place of work, in case they are needed overnight, should be paid the minimum hourly wage.
The Royal Mencap Society is challenging a tribunal decision made in favour of Claire Tomlinson-Blake, a Mencap support worker in Yorkshire.
The court was told on Tuesday that Mrs Tomlinson-Blake received a salary for her full-time job helping vulnerable adults living in their own homes, and sometimes had to work a sleep-in shift between 10pm and 7am.
For those shifts she was paid an allowance of £29.05, which included pay for an hour's work, and if she was woken in the night and had to work for more than an hour, she would receive extra pay for the time she had worked.
However, the Employment Tribunal found she used her "listening ear" and her experience to know when she was needed, and was "working" even when she was asleep.
The tribunal said she was therefore entitled to receive an hourly minimum wage, which would have been more than £60 per shift, a decision upheld by the Employment Appeal Tribunal (EAT) last year.
David Reade QC representing the Royal Mencap Society said: "There were no specific tasks allocated to Mrs Tomlinson-Blake during a sleep-in.
"She was allowed to sleep, and further, there was an expectation that she would have had a sufficiently good night's sleep to enable her to perform her duties satisfactorily the following day.
"She was obliged to remain at the premises in question throughout the sleep-in."
Mr Reade told the court that, in a 16-month period, Mrs Tomlinson-Blake only had to get up during the night six times.
The appeal is being resisted by Mrs Tomlinson-Blake.
The other case being considered involves a Surrey care home worker who failed to convince the (EAT) he should have been paid minimum wage for shifts when he was "on call".
Care England, the body which represents independent care providers, said the case could cost the sector £400 million in back-dated pay and £200 million a year from 2020 if the court rules workers should be paid the minimum wage.
And lawyers for the Local Government Association, which represents councils, said its members would face "very significant unfunded historic liabilities" if Mencap's appeal is not allowed.
The hearing is due to continue on Wednesday, following which Lord Justice Ryder and two other judges are expected to reserve their ruling.