Controversial plans to cut council tax relief in Bristol are "potentially illegal" local politicians have said.
Bristol mayor Marvin Rees has said the council can no longer afford to run the Council Tax Reduction Scheme.
The claim was made by the Liberal Democrat and Green party groups at Bristol City Council.
The council is asking for views on plans which could see some 25,000 households paying between £300 and £600 more per year.
The BBC is awaiting comment from Mr Rees regarding the claim.
The reduction scheme is a means-tested benefit to help people on low incomes.
Leader of Bristol's Liberal Democrat Group Gary Hopkins said: "We believe there's a severe danger the way he's done that is illegal and could lead to a terrible mess in Bristol if a legal challenge comes in and the whole thing collapses."
He added he had written to the mayor outlining his concerns and how another council, Haringey, attempted the same approach but was blocked from doing so after a judicial review.
Carla Denyer from the Green party described the proposed cuts as "immoral".
She added: "They would inevitably see visits from bailiffs imposed on the poorest in Bristol, as those already under immense pressure due to cost of living increases are simply unable to pay.
The Greens called on the mayor to re-draw the consultation options.
Campaigners have previously warned the cuts would put more people "on the bread line".
Bristol is one of the last councils to maintain a high level of support for low-income households.