Ministers have been warned the clock is ticking for them to prevent the collapse of care providers for people with learning disabilities.
Mencap said the Government had until August 15 to halt HM Revenue and Customs enforcement action over demands to fund £400 million in back-pay for sleep-in carers.
The situation is the result of a change in guidance on whether care staff should be paid the minimum wage while they are asleep but on call to deal with an emergency.
The charity, along with other care providers United Response, Voyage Care and Hft, met ministers on Thursday to address the issue.
But they said the Government had not put forward a solution or indicated when a decision on the back-pay demand would be made.
Mencap chairman Derek Lewis warned that failing to underwrite the cost of the back-pay liabilities could lead to the collapse of providers looking after 178,000 people with serious learning disabilities, which would have a serious knock-on effect on the NHS.
He said: "The learning disability sector welcomed the acknowledgement by junior ministers of the gravity and scale of the problem and their concern for vulnerable families.
"However, the inability of Government even to indicate the likely timing of a decision let alone put forward an interim solution to the problem, was more than surprising after months of discussion.
"As Mencap has made clear, the clock is ticking - no decision to halt HMRC enforcement action by 15 August will have very serious consequences for providers and their continued viability to operate.
"Mencap will continue to press Government to prioritise the future care of some of the most vulnerable in our society. Stopping the sleep-in crisis should be a top priority for the senior ministers in any caring Government."
When the national minimum wage (NMW) was introduced in 1999, the Government advised that time spent asleep did not count as "work time", so a flat-rate "on call" allowance has been the norm across the sector.
But following two employment tribunal decisions, the Department for Business, Energy and Industrial Strategy (BEIS) published new guidance in 2016 saying the NMW - and now the national living wage - should be paid for sleep time.
A Court of Appeal ruling on the interpretation of the law is expected in March 2018.
Mr Lewis said: "The learning disability sector needs Government to face up to its responsibilities and give a public undertaking to fund back-pay if the Court of Appeal upholds the tribunal decision."
Trade union Unison has argued that charities and other care providers should pay the minimum wage for sleep-in shifts but called for the Government to properly fund the social care sector.
The Government allocated £2 billion to the sector in the Budget and says the guidance on sleep-in shifts reflects the case law but has promised that any action taken would be "fair and proportionate".
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