Health and social care watchdogs in west London have had their budgets halved, sparking fears patients' voices will be muted.
Healthwatch, which is led by patients using local authority funding, was set up to ensure the needs of local populations across the country are being met.
In Hounslow, the local group has seen its annual budget for this year slashed by 53.4%, from £191,611 to £89,378.
This is the biggest cut by any local authority, according to Healthwatch England, the independent consumer champion supporting the network of local groups.
They are among a third of local authorities nationally to reduce the budget, and just eight to do so by a fifth or more - a move Healthwatch England claims could weaken patients' voices, potentially affecting the standard of care available.
Anna Bradley, chair of Healthwatch England, said: "On average, local Healthwatch groups across the country receive less than the cost of a single first class stamp per person to spend on ensuring the views, experiences and needs of the public drive change in how services are delivered...
"We urge those councils that have decided to impose such severe cuts to outline why they have made this decision and to provide the public with the voice they need to influence the big decisions to come about how local health and care services are delivered."
Healthwatch England said it had now written to the 10 local authorities imposing the largest cuts and asked them to outline how they will assure the local group is able to deliver its statutory duties within the reduced budget.
It said it recognised cuts to their own budgets meant local authorities were under increasing pressure to find savings but it was concerned at the scale of cuts by some councils.
Councillor Kamaljit Kaur, Hounslow Council's cabinet member for health, said: "We value the function and ethos of Healthwatch but we are disappointed with the performance of the local group.
"So, in discussion with the local Healthwatch Board, we have asked them to focus on a number of jointly agreed priorities and reduced their funding accordingly.
"We have advised them that in the event that they meet these targets to the satisfaction of ourselves, the CCG and the Health and Well-being Board, we will consider extending their role and funding."
A Harrow Council spokesman said: "Harrow is completely committed to Healthwatch's aims and our priority is to deliver a sustainable future for Healthwatch here for years to come.
"After a full year of negotiations with our local Healthwatch, against a background of austerity cuts, we agreed on annual funding that will not just continue to deliver the service here in Harrow, but improve it.
"We are confident the funding is ample – having just completed a competitive tender for the work, with three existing providers keen to take on the service at the new price."
A spokeswoman for Ealing Council said: "Local councils have been hit harder than any other part of the public sector by central government cuts. During this decade we expect the money we have to spend on services to roughly half. This will mean the council will need to make £97million of cuts by April 2019, in order to meet our legal duty to balance our budget.
"The combined pressure of insufficient funding, growing demand for social care and escalating cost means we have no option but to make tough decisions.
"We are doing everything possible to protect our most vulnerable residents, including funding a number of organisations that give people who use health and social care services a voice, such as Healthwatch.
"We will continue to work closely with these organisations to ensure that services continue to support people who rely on them and are provided as efficiently as possible within the budgets available."