Leading charities have called on David Cameron to take "bold" action to tackle the growing pressures on health and social care, warning that vulnerable elderly and disabled people will "bear the brunt" if he fails.
A letter backed by nearly 40 organisations, including older people's charity Independent Age, Macmillan Cancer Support and Marie Curie, urges the Prime Minister to set up an independent commission to review the system.
It warns there is "no room for complacency" and points to official figures that suggest nearly a quarter of the population will be over the age of 65 in just over 20 years' time.
The letter states: "We need to ensure we have an NHS and social care system that is fit for purpose otherwise it is the elderly, disabled people and their carers who will bear the brunt of inaction.
"Bold long term thinking is required about the size, shape and scope of services we want the NHS and social care to provide - and an honest debate about how much as a society we are prepared to pay for them.
"It is vital that you meet the challenge posed by an ageing society, and an underfunded care system, head on and establish a cross-party commission to review the future of health and social care in England."
It comes after former health minister Norman Lamb warned some experts believe there will be a £30 billion "gap" in NHS funding by 2020 despite the Government already committing extra cash.
The Liberal Democrat, backed by former health secretaries Stephen Dorrell and Alan Milburn, put forward proposals for an independent commission to review the future of the NHS and social care.
It would take evidence from the public, health staff and other interested parties with the aim of proposing a new funding settlement
Simon Bottery, policy director at Independent Age, said: "Without a robust health and care service that delivers for older people when they need it, the UK will never be truly prepared for ageing.
"A commission on the future of health and social care is the vital first move towards recognising that the health and care systems cannot work in isolation - only when they work effectively together can the needs of older people be met.
"This is a conversation we cannot avoid if we are truly committed to ensuring older people have the quality of life they deserve. We urge the Prime Minister to back this commission."
David Sinclair, director of think tank the International Longevity Centre, which examines ageing and population, said: "The UK is facing dramatic demographic change - in the next 20 years the number of people aged 85 and older will more than double to over three million.
"It is crucial that we are prepared for that change. We need to start talking now, honestly and openly, about what standards of health and care older people can expect now and in the future."
A Department of Health spokesman said: "We know the NHS is facing challenges as our population ages, but we are absolutely committed to its future and are investing £10 billion more to fund the service's vision for the next five years.
"Since 2010, we have employed 10,100 extra doctors and 8,500 extra nurses and by 2020 the NHS will be a truly seven-day service offering the same world-class care every day of the week."