The RCN is campaigning for guaranteed access to specialist nursing care for all patients with long term conditions.
The NHS could save millions of pounds by investing in nurses to support people with conditions like Parkinson's and multiple sclerosis, a union said.
Specialist nurses help keep people out of hospital by offering advice on medication and day-to-day living with an illness, the Royal College of Nursing (RCN) said.
It estimates £56 million a year could be saved on care for people with Parkinson's through greater use of specialist nurses. Meanwhile, £180 million could be saved by treating multiple sclerosis flare-ups at home rather than in hospital.
Another £84 million could be saved if specialist nurses supported people with epilepsy rather than relying on GPs.
The RCN surveyed almost 300 specialist nurses working in 60 NHS organisations and charities and found only 36% believed all those patients who needed specialist nursing currently received it.
Of the 49% who identified problems accessing specialist care, 69% said specialist nurse services are overloaded and cannot take on new patients. More than a third said they had seen cuts in services over the last 12 months while 57% are concerned jobs will be threatened in the near future. Most (95%) of those seeing cuts said they were within the NHS.
Specialist nurses work in a range of areas, including cancer, diabetes and asthma.
Dr Peter Carter, general secretary of the RCN, said: "Nurses realise that whoever wins the next election will be looking to make savings and to deliver more for less.
"While the temptation may be to cut or downgrade specialist nursing roles, this would be a false economy which would only add to the growing cost of treating long-term conditions. In fact, specialist nurses save money through the better management of conditions, keeping patients out of hospital, and advising on the best drug and other treatments."
The RCN said the NHS was in such debt in 2006 that many specialist roles were lost, frozen or downgraded.