Unison has lodged an application for permission to appeal to the Supreme Court in the Mencap case.
It is expected to take up to eight weeks for the court to decide whether to grant permission to appeal.
In July, the Court of Appeal decided in favour of Mencap – that it is only time spent awake and working which is counted during a sleep-in shift.
However, Unison, which initially took the case to an employment tribunal on behalf of care worker Clare Tomlinson-Blake, has decided to appeal last month’s judgement.
Its grounds for appeal are that the previous approach to the law was correct, the Low Pay Commission recommendations have been misunderstood, and the Court of Appeal has contradicted itself.
Responding to the news, a Mencap spokesperson said: ‘Any appeal will plunge care providers, care workers and the people they support back into uncertainty. It is disappointing that government has not committed to legislation so that this issue is clarified for the future.’
Professor Martin Green, Care England chief executive, said: ‘It is disappointing that unison has decided to appeal the judgement in the sleep-in case. If their appeal is successful many of their members will find themselves without a job because a lot of services will go into liquidation.’
Unison took the initial case to an employment tribunal on behalf of care worker Clare Tomlinson-Blake. It argued that sleep-in shifts should count as working time, and should be paid at hourly minimum wage rates or higher.
Its head of legal services Adam Creme said: ‘There will be a period of time when the Supreme Court considers the application, but it is reasonable to expect the court will agree to hear an appeal.
‘Assuming this is agreed, Unison will be taking the appeal forward and fighting for our members.
‘We believe the Court of Appeal got this decision completely wrong and will do everything we can to reverse it.’
If permission is granted, the case is expected to be heard in late 2019.