David Cameron has been warned by a group of leading charities that pensioners could face a life of "misery and fear" without reforms to social care.

A coalition of 78 charities and campaign groups have told the prime minister in an open letter to the Daily Mail that a postcode lottery of access to care is leaving many elderly people in "quiet desperation."

The campaign comes at the same time as new charity, the Carers Trust, was launched to help those struggling to look after sick or disabled family members.

Thousands of unpaid carers are suffering health and career problems because of the lack of help available, the charity has said.

Almost six out of 10 of those polled by the Carers Trust said the strain of looking after a loved one had affected their mental health while the same amount said it had harmed their working life.

Around two-thirds (64%) said they had never accessed any support or services such as counselling or respite breaks while six out of 10 carers who had been looking after someone for more than five years had never accessed any additional support.

There are approximately six million carers in the UK looking after unwell or disabled friends or family, the charity said.

Many find the physical and mental strain of caring has a huge impact, leaving them feeling isolated and without a place to go to seek help and advice.

Anne Roberts, chief executive of Carers Trust, said:

"We've launched Carers Trust so we can ensure that all carers know where to go to get that help when they need it and to help society recognise and value the role of carers in our communities across the UK."

The Princess Royal, who is president of the charity, said: "Carers Trust will provide a united and stronger voice for unpaid carers which will enable us to continue to raise awareness of carers' issues with government, other policy makers and the general public, and hopefully increase funding opportunities to develop and deliver the services so needed by carers and those they care for."

Under the current system, pensioners have to pay the cost of their own care if they have savings or assets worth more than £23,500.

The charities have pleaded with Mr Cameron to make social care reform his "personal mission" and said there was widespread support "across society and the political spectrum".

The letter and the launch of the charity comes ahead of this week's Queen's Speech, with some groups fearing it will not include a Bill on social care.

The letter, which has been signed by Age UK, Saga and the Local Government Association, reads: "Social care is in crisis - the system is chronically underfunded and in need of urgent reform.

"Without this, too many older and disabled people will be left in desperate circumstances: struggling on alone, living in misery.

"The system is a lottery - some of us will be lucky enough never to need care, but there are many of us who need support at some stage in out lives to carry out everyday tasks and could lose everything: our savings, our dignity, our independence.

"That is why we are calling on you to take forward social care reform as your personal mission."

A Department of Health spokesman said: "We absolutely agree that the social care system is in need of reform.

"We have worked with people, including care providers and charities, to see what changes they want made in care and support.

"Their feedback - more than 600 formal responses - has shaped the forthcoming White Paper. This will make sure we create a sustainable system that will mean people and their carers get the quality of care they want.

"In the Spending Review, the government recognised the pressures on the adult social care system, and took the decision to prioritise adult social care by allocating an additional £7.2 billion to the system over the four years to 2014/15.

"We will publish our White Paper on care and support shortly and are working hard to secure cross party agreement to find a sustainable long-term solution on social care funding."

A White Paper on long-term care will be published in June, but it will focus on the quality of care provision, with the issue of paying for it relegated to a "progress" document.

Carers can get help by visiting their local Carers' Centre or Crossroads Care Scheme or visiting www.carers.org.