Care home residents are suffering because they and their families are not told about important parts of their contracts before they sign, an investigation has found.
Which? said care homes could be breaking the law by neglecting to inform residents and their families about any part of the contract deemed necessary to the decision-making process.
Any care home that fails to provide prospective residents and their families with important information they need to make an informed decision risks being in breach of the Consumer Protection from Unfair Trading Regulations.
The watchdog contacted 50 homes requesting documents and a sample contract on the premise of looking for care for an elderly relative, but fewer than 10 provided the information.
Of the four contracts received, three included terms that could be considered unfair to residents, including charging fees for a month after death and the right to terminate a contract with 24 hours notice for undefined "detrimental behaviour".
The other 46 homes refused to send a sample contract, often referring investigators to visit the home instead or directing them to websites that did not provide the information.
The consumer group also studied reports from more than 500 members of the public about their experiences with care home contracts, finding that just 54% said the provider checked if they understood the document they signed.
Some 27% said no-one from the care home checked if they understood the terms and conditions, while 19% said they did not know if checks were made.
Of those who said no checks were carried out, 31% said they did not understand the contract at all or did not understand it very well.
Although most - 81% - said they understood their contract, 38% did not know if their care home could evict a resident without giving a reason and 34% did not know whether the home could evict a resident without giving the notice period stated in the contract.
Which? said it believed the lack of transparency left potentially vulnerable people open to exploitation.
Cases reported to Which? included a manager placing a resident with dementia on 24/7 one-to-one care at a cost of £15 per hour, sending bills from £4,000 a month to more than £15,000, with family told that they had signed an agreement saying this could be done.
Another reported being served an eviction notice without explanation and the home manager refusing to discuss it, but having to finally agree that the contract has a clause stating that residents may be asked to leave and explanation need not be given.
There are also widespread reports of grieving families being charged after-death fees, often totalling several thousands of pounds, to cover lost income while rooms are cleared out and cleaned before another resident can move in - a practice currently under scrutiny by the Competition and Markets Authority (CMA).
Which?'s findings come days before a deadline for the Government to respond to the CMA's damning care home market study which urged action on unfair contracts, fees and evictions.
Alex Hayman, Which? managing director of public markets, said: "It's unacceptable that care homes are making it difficult for people to get hold of contracts and the terms and conditions they are signing up to when making such an important life decision.
"Far too many care home residents are hit with unexpected fees or contract terms, which can have far-reaching and devastating consequences for vulnerable people and their families at an already distressing time.
"The Government must now ensure the rules governing care homes are fit for purpose and adequately protect residents and their families in its upcoming response to the competition authority's market study."