Health service chiefs are to tell the government it must guarantee spending on social care as well as on hospitals if the NHS is to ride out five more years of austerity.

In a poll of more than 300 top managers and directors of NHS care bodies, 99% have warned that cuts to social care funding are loading extra pressure on the health service and 92% say such cuts are hitting their own organisations.

Asked if they would support a binding agreement on social care spending as well as on health, 86% said yes – more than those supporting a binding agreement on NHS spending alone.

The unprecedented findings will be published later this week as NHS leaders gather for the health sector’s main annual conference. The health secretary, Jeremy Hunt, is due to address the NHS Confederation event on Thursday.

Directors of adult social care services in English councils will on Thursday release details of their annual spending survey, which is expected to show further deep cuts being made this year in care and support for elderly and disabled people.

Last year’s survey found that social care budgets had been cut by 12% in cash terms under the coalition government, while demand for services had grown by 14%, largely because of the ageing population. Councils are forecasting a £4.3bn black hole in social care funding in England by 2020.

Without care and support to keep them living in their own homes, people with care needs are turning up in hospitals where two-thirds of beds are already occupied by elderly patients.

The poll of health leaders was carried out by Populus for the confederation, which represents bodies both commissioning and providing services. Of 313 respondents, 99% agreed and no one disagreed with the proposition that social care cuts were increasing pressure on the NHS as a whole, while 92% agreed and only 2% disagreed that they were increasing pressure on their own organisation.

In a previous poll for the confederation in 2012, 66% of respondents said social care cuts had affected their organisation over the previous 12 months – suggesting a sharp deterioration in the picture since then.

This time, 79% of all those reporting an impact said patients were forced to stay longer in hospital because of lack of social care support on discharge. Other reported effects included higher demand for community health services (69%), mental health services (61%) and GP services (57%); growing numbers attending hospital (67%), being admitted (66%) and attending A&E (61%); and more emergency readmissions to hospital (59%).

The poll found 86% of respondents support, and just 6% oppose, the idea of “a binding agreement on absolute NHS and social care spending over the course of the next five years”. On such an agreement on NHS spending alone, support was 81%.

The new government has promised to increase real-terms NHS spending in England by a minimum £8bn a year by 2020. But social care funding, which is channelled through local councils, is exposed to a further 15.7% cut in the budgets of unprotected Whitehall departments as indicated by analysis of government plans by the Institute for Fiscal Studies.

Dr Johnny Marshall, director of policy at the NHS Confederation, said “central government needs to provide a strong commitment to solve and stabilise social care funding for the future”.

A Department of Health spokesperson said: “We have given an extra £1.1bn to councils to help protect social care services this year and have committed £10bn extra by 2020, which is going into health and social care systems that are being merged for the first time.”