Momentum Care – which is part of Ireland's Rehab Group – will end deals with local authorities in Glasgow, South Lanarkshire, Moray, Aberdeenshire and South Ayrshire at the end of the financial year.
The charity said it was making "every effort" to ensure a smooth transition to new providers, but some of its clients fear they may now have to get used to new carers.
One relative of a Momentum client said: "If the transition is not smooth then it is going to be very difficult for all the family members and the people cared for.
"My father is looked after by staff from Momentum – some of them are more like friends than carers and regularly pop by even outside their working hours."
Momentum said it had informed the relevant councils as long ago as October and councils are understood to be trying to make alternative arrangements.
A spokesman for the charity said: "Momentum Care and local authorities have made contact with all service users, their families and carers to inform them of this development, to reassure them of their commitment to ensuring a smooth transition and ongoing support, and to discuss with them their future service provision.
"Momentum Care has taken the decision with regret to cease delivery of these particular services as it was no longer financially viable for the organisation to do so. A number of other Momentum Care services will continue unaffected."