Vital home care services for the elderly and vulnerable could be disrupted, regulators say.
The Care Quality Commission has issued a notice saying it has serious doubts about the future of Allied Healthcare.
The CQC said it was concerned about its prospects from the end of this month, but the firm said the move was "premature and unwarranted".
The company provides services, such as help washing and dressing, to more than 13,000 people across the UK.
The CQC just has responsibility for England, where 9,300 people rely on Allied Healthcare services across 84 council areas, more than half the total.
The regulator has written to all the affected local authorities as they would have responsibility to step in if services are disrupted.
Financial difficulties
Allied Healthcare announced earlier this year it was struggling with debts blaming low fees paid by councils, but it did manage to restructure its finances on that occasion.
But the CQC has now said the company had only be able to confirm it had funding in place until 30 November.
The regulator said it had a legal duty to warn councils that business failure was likely and services could stop as a result in its view.
It does not mean failure will definitely happen, just that as things stand it is deemed likely.
The BBC understands there has already been an offer made from a new lender that could secure the firm's immediate future.
This is the first time a notice like this has been issued to a whole provider – previous warnings have just related to individual services.
Andrea Sutcliffe, from the CQC, said: "I understand this is a very unsettling time for everyone who uses Allied Healthcare's services, their families and loved ones, and staff.
"We will continue to work closely with Allied Healthcare and all of our partners to make sure appropriate action is being taken in the interests of people's continuity of care if this proves necessary.
"It is of course possible that the company is able to avoid service disruption, and if that is the case, we will revise our position accordingly."
A Department of Health and Social Care spokesperson said there was currently no disruption to any services provided by Allied Healthcare, and that local authorities were being given time to ensure continuity of care for people using services.
But a spokesman for Allied Healthcare said the company was "surprised and deeply disappointed" by the move.
He said it was "premature and unwarranted" as there was no risk to services because financing had been secured.
"The CQC has disregarded these assurances in spite of the robust evidence we have provided," he added.
The news comes just weeks after the UK Homecare Association, the umbrella group for providers, warned services were being put at risk because councils were trying to get care "on the cheap".
Its report said only one in seven local authorities were paying a fair price for care, forcing a number of firms to pull out of contracts.
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