Spent properly, the fund could have allowed 33,000 carers a three-hour break every week, or given 173,500 carers an annual week's holiday, according to the strategy's projections.

Stephen O'Brien, shadow health minister, described the findings of the investigation as "an absolute scandal" and a betrayal of some of Britain's most vulnerable people.

Charities say they believe the pattern will be repeated with the remaining £100 million funding allocated for 2010/11, following indications they have received from NHS primary care trusts (PCTs).

The £150 million pledge was the centrepiece of the Government's strategy for carers, announced in 2008, when ministers vowed to double the amount of respite care available, in response to pleas from charities.

All PCTs were given a share of the funding for two years.

The research - by two charities, The Princess Royal Trust for Carers and Crossroads Care - shows that in 2009/10, just 23 per cent of the funds intended for respite care were spent on such support.

Most PCTs devoted just a fraction of the new funds to carers, while nine did not spent a single penny of their allocation on such help.

Carole Cochrane, chief executive of The Princess Royal Trust for Carers, said: "We are appalled that even this tiny pledge to the ordinary people who keep the NHS afloat with their vast caring contributions could not be honoured."

The charity is inundated with calls and visits from carers who have reached the end of their tether, she said.

"Just recently we were contacted by a woman of 79 who has a sick husband and a grown-up daughter with profound learning disabilities.

"Her husband had a heart attack in the middle of the night, but she couldn't go with him in the ambulance because no help could be found," said Mrs Cochrane. "These situations are desperate.

"Then there are youngsters who come home from school every day to bath, cook and provide intimate care for their parents.

"They often end up with a maturity beyond their years but they also need a childhood, and to have some time free to play and do normal activities for children of their age," she added.

"Without these breaks carers can be left to reach breaking point."

Imelda Redmond, chief executive of Carers UK, another charity, said: "We are absolutely furious about this. The Government made a huge fanfare about the strategy and it has amounted to so little.

"Respite care is the number one thing carers want; it can allow them to carry on, in many cases preventing an elderly person from having to go into residential care, and ensuring that husbands and wives are not forced to live apart.

"Nothing could have a greater impact on carers than this failure."

The Conservatives described the findings as "yet another broken promise" from the Government.

Mr O'Brien said: "Carers work tirelessly, and often thanklessly, looking after friends and family members, and deserve the support of the public, and of government.

"It is an absolute scandal that this money that Gordon Brown promised them is apparently being soaked up in NHS bureaucracy."

The nine PCTs that admitted they had not spent a penny of the funds allocated for respite care on services for carers in 2009/10 were: Cumbria, Hartlepool, Kirklees, North Tyneside, Northumberland, Portsmouth, South Gloucestershire, Stockton-on-Tees and Worcestershire.

On Tuesday, the Princess Royal, founder of the charity, will host a summit bringing all three main political parties together to discuss the crisis in care of the elderly and vulnerable, and the role played by Britain's six million carers.

Labour, Conservative and Liberal Democrat health spokesmen have all agreed to attend the talks, despite the fact that the Tories have previously refused to take part in cross-party discussions about funding for care of the elderly, following Labour's refusal to rule out a "death tax" on properties to fund long-term care.

The charity will urge all three parties to make a manifesto commitment giving all carers an entitlement to respite care.

Research has estimated that the free care provided by voluntary carers, usually relatives, saves the state up to £87 billion each year.

With Britain's population ageing rapidly, projections suggest that the number of people caring for relatives will increase by 50 per cent within a generation.

A spokesman for the Department of Health said it was down to the local NHS to decide how to spend the money, in the light of local needs.

The NHS Confederation PCT network, which represents the 150 PCTs, said the money was not "ring-fenced" and that organisations had to balance their priorities.

Joyce Toyn, 75, from Brenchley, Kent, cares for her husband Arthur, 83, who was diagnosed with Alzheimer's disease and cerebral vascular dementia three years ago.

As he became increasingly confused, and reliant on his wife, she struggled to cope with the burden, until, last December, she found a local charity which now sends a carer to provide respite cover for one morning each week.

Mrs Toyn said the help she receives has helped her to "keep her head above water".

"It stops me drowning in my sorrow," she said. "Arthur often doesn't know who I am.

"He will tell me I am beautiful, then a moment later he will say his wife has died, or that he doesn't know where she has gone, and get very anxious. I feel like I am already mourning my husband but he is still here.

"Sometimes it gets on top of me: I can get lonely, and feel trapped, and the responsibility becomes overwhelming. A few hours out of the house, a bit of exercise, or talking to someone else can make a huge difference."

Kelly Walker, 16, from Crystal Palace in south London, has looked after her mother Claire since the age of 10.

Her mother, a single parent who suffers from spinal problems, needs help washing, getting around the house, and in and out of bed.

The sixth-former said she is used to not having the freedom of other teenagers, but says that respite days out run by a local charity, which allow her to meet other teenagers with caring responsibilities, make a huge difference.

"I used to get very upset, because seeing mum in so much pain is very difficult, but I have got used to it," she said.

"My friends try and understand, but my life is quite different to theirs, and I can't do the same things they can, so it makes a massive difference if I can just chat to people my age in the same situation."