The Government was warned of a "crisis" in care for the elderly on Sunday, because of widespread closures and cuts hitting day centres.
Unison said ministers should ensure that local councils are given the funding they need to keep day care centres running. Closing them was described as a "false economy" because they provide much-needed respite for carers, as well as monitoring and improving the physical and mental health of users.
A survey of social care workers in more than 100 areas of England, Wales and Scotland for the union by the University of Birmingham's health services management centre showed that 57% have seen day centres close down.
Services for elderly people were hardest hit, followed by those for people with learning and physical disabilities, said Unison.
More than half said they were aware that more changes will be made in the future, as deeper cuts to social care budgets are made.
Two-thirds reported increasing charges for attendance, meals and transport, with some centres stopping the provision of fresh meals entirely. Many of those surveyed said that fees had risen "substantially", often going from a minimal charge to as much as £50 a day.
Heather Wakefield, Unison's head of local government said: "The coalition's cuts and austerity agenda are hitting some of the UK's most vulnerable people and another round of budget cuts means this situation will only get worse.
"For elderly people, day centres guard against loneliness, their loss is devastating. Research has revealed loneliness to be as deadly as smoking, alcohol or obesity for a person's health, so these cuts could cost lives. The cuts are also a false economy as more elderly people will be forced into long-term care, or into A&E - a far more expensive option for the taxpayer.
"For a young person with learning or physical disabilities, a day care centre can be their main chance to build friendships and an enriching social life. As cuts close them down, families will be left to pick up the pieces, but they too will miss out on the vital respite that day centres can bring from their caring responsibilities.
"The Government needs to face up to the crisis in care and ensure that local councils are given the funding they need to keep day care centres open. Spending cuts are not inevitable. There are other choices that the Tory-led coalition could, and should, be taking to protect some of the most vulnerable people in our society."
Dr Catherine Needham, of the University of Birmingham's health services management centre, said: "Reduced access to day centres reflects the pressures of introducing personalised funding streams at the same time as making massive cuts to social care budgets.
"The commitment to choice and control within the personalisation agenda is absolutely right: no-one should be made to attend a day centre if that is not what they want to do. But choice may be taken away from service users if a valued day centre is closed, particularly when the alternative is more time spent at home.
"Personalisation can't be an agenda for empowerment and inclusion without a funding settlement that preserves both private dignity and public spaces."