Complaints about English local councils' handling of tax and benefits soared by more than a quarter last year, a standards watchdog said.
The Local Government Ombudsman said they went up by 26% and also reported a 16% rise in disputes over the provision of social care.
Almost half of cases in both areas (49% and 48%) were upheld, it reported, complaining that authorities too often failed to consider how vulnerable complainants might be.
Town hall chiefs said the increases were the direct result of central government slashing councils' funding while asking them to do more.
But the Ombudsman's report said it had "no evidence to indicate whether or not there is a link".
"Some people who complain to us about benefits and tax rely on these council services for their everyday wellbeing, therefore failures to administer properly can have an acute impact on their lives," the watchdog said in a report compiling national statistics for the first time.
"A common fault we see from councils is a failure to consider whether the person complaining may be in a vulnerable situation," it noted - as well as people not being informed of their right to appeal.
On care, it added: "A common issue is the failure to monitor properly and record the condition of people and the care provided which leaves family members uncertain that loved ones are properly cared for in their absence."
A slight drop in the overall number of new complaints - 18,436 in 2013/14 compared with 18,940 the previous year - masked significant increases as it included a 39% fall in housing complaints after that policy area was handed to a separate watchdog, the Ombudsman said.
It also criticised as a "missed opportunity" the 20% success rate of complaints brought back for adjudication despite councils being given a chance by the watchdog to resolve them.
Peter Fleming, chair of the Local Government Association's improvement and innovation board, said: "It is unsurprising that issues around council tax and adult social care are areas of concern for complainants as this is where there has been increasing pressure on budgets and demand for services.
"When government handed councils the responsibility for administering council tax support, it cut hundreds of millions in funding for it. This has left councils facing an impossible dilemma of having to ask those on lower incomes to pay more council tax or make bigger cuts to local services like filling potholes, collecting bins or caring for the elderly.
"Councils have also been forced to reduce adult social care budgets by £3.5 billion over the last four years but have managed to limit the impact on the essential care services that people rely on. However, it is now inevitable that services will start to suffer unless there is a long-term commitment to reforming our broken adult social care funding system."
He went on: "Local government is one of the most trusted parts of the public sector. Public polling has consistently shown that satisfaction with councils has remained consistently high despite a challenging financial climate and rising demand."
Research carried out by the Ombudsman found 56% of people did not recall being made aware of the local authority's complaint handling procedures and 43% were not advised that they could refer their complaint to the watchdog.
Ombudsman Jane Martin said: "We are releasing this data so that the public has a better understanding of the role of the Ombudsman and the work we do.
"We also hope the information will support better local accountability by helping people make informed choices about local public services; helping councils reflect on their complaint handling; and supporting local scrutiny.
"It is important that the public has a clear route to redress when things go wrong. We want to work with councils to support excellent local complaint handling so that things are put right as quickly as possible.
"Our research highlights areas where improvements could be made to ensure the complainant's journey runs as smoothly as possible."
Local government minister Brandon Lewis said: "Research shows the public think councils are delivering better services and value for money than before 2010.
"However, councils have a duty to protect those most in need of support and this report shows that more can be done. That is why this Government has been working to give families greater financial security.
"We have set aside an extra £3.8 billion for new adult social care services and given extra funding to town halls to help freeze council tax.
"We have handed local residents new rights to veto big local tax hikes, so local people have the final say on the amount they pay. However, council tax benefit doubled under the last administration costing every household £180 a year so welfare reform is a vital part of reducing the inherited deficit.
"Localising council tax support gives councils stronger incentives to cut fraud and support people to get back into work."