A "postcode lottery" has local authorities in the south contributing far more to the cost of long-term care than in the north, new figures show.
Which? submitted Freedom of Information requests to 180 local authorities across England, Wales and Northern Ireland and found "huge variations" in the standard weekly rate councils will contribute to the funding of residential care.
In general, authorities in London and the south of England provided the highest rates, while the lowest were in the north.
However, areas in close proximity could vary widely, with a difference of £138 for the standard weekly rate between neighbouring Greater London boroughs Bromley (£555) and Croydon (£417).
The study found around a third of councils (36%) have a maximum standard rate of £434 for personal care, more than half (53%) give a maximum of £435 to £539 and 11% give a maximum of more than £540.
The highest rate found by Which? was Lewisham's maximum of £768, while Blackburn and Darwen gave a rate of £357.
Around half of care home residents have part or all of their fees paid by the local authority.
But many people top up the contribution themselves, and could face potentially high bills in areas where the local authority pays a lower proportion of the full costs, Which? said.
The watchdog also found significant differences between the standard rate some councils pay and what the cost would be for those self-funding.
In Exeter the council pays £442 to £471 a week, while the fees Which? saw for self-funders were £300 to £1,200.
Which? executive director Richard Lloyd said: "Understanding the options available for long-term care can be a minefield, particularly with such huge variations in the funding available.
"People looking to make difficult decisions about care should use free, independent sources of advice to help them find the information that's relevant to their situation."