Small charities and faith-based groups are struggling to meet the needs of rising numbers of destitute migrant children and families, according to a report.
The organisations are stepping in to provide housing, food and clothing, the study said.
But it warned that the voluntary sector faces a lack of capacity while demand for services increases.
The report, published by Oxford University's Centre on Migration, Policy and Society, said: "The policy environment has contributed to this dynamic, with migrants increasingly restricted from accessing mainstream benefits, thus relying more and more on the voluntary sector, communities and local authorities for support; and at the same time funding for local authorities and for voluntary sector organisations has dwindled."
Researchers carried out interviews and focus groups with representatives of 51 voluntary sector and faith-based organisations and funding bodies.
They found that almost all participants providing destitution support for migrants said that their organisations could not meet demand for their services.
One organisation in London was described as having queues outside its door from 6am.
The context in which voluntary sector bodies are providing destitution services to migrant children and families has changed significantly in recent years, according to the report.
It said that in addition to increased numbers of migrants living in the UK, their entitlements to welfare benefits have reduced, meaning that they are increasingly reliant on voluntary sector organisations, communities and local authorities for basic support.
Report author Jonathan Price said: "Small charities and faith groups are stepping in to provide vital support to destitute children and families such as housing, food and clothing as well as help accessing services.
"Often they are having to provide this help with little or no financial assistance from the state, whilst negotiating complex and frequently changing legal contexts, and situations where children and adults are at risk."
A Government spokeswoman said: "We provide support to those who genuinely need it where they are living legally in the UK.
"We also provide support to asylum seekers who would be otherwise destitute.
"We expect anyone who has no legal right to remain in the UK to make arrangements to leave voluntarily and offer help for them to do so."