People suffering long-term illnesses with a high level of pain are not receiving the relief they need from doctors unsure when to prescribe strong drugs, a medical body said.
Patients suffering from chronic or incurable illnesses are also exacerbating the problem because they are wary about long-term use of opium-derived painkillers because of possible side-effects and addiction, according to the National Institute for Health and Clinical Excellence (NICE).
It is issuing new guidance to ensure "safe and consistent" prescription of opioids such as morphine, which is said are often the only adequate form of pain relief for people suffering from diseases and disorders such as cancer, heart failure and motor neurone disease.
Professor Mark Baker, director of Nice's Centre for Clinical Practice, said: "Many people with chronic or advanced conditions will experience a high level of pain which can only be treated by opioids such as morphine as weaker forms of pain relief will no longer be effective.
"However, we understand that patients can be anxious about using these medicines for a number of reasons. Likewise, healthcare professionals may not always be sure about when to prescribe certain types of opioids. The new guideline aims to address all those fears and provide clear advice to the NHS to ensure a consistent approach to treatment and ultimately help to drive up standards of care."
Nice said that each year 300,000 people are diagnosed with cancer in the UK and it is estimated that there are 900,000 people living with heart failure. Others live with chronic illness such as kidney, liver and respiratory disease, and with neurodegenerative conditions.
The guidelines focus on talking to patients suffering high levels of pain about their concerns and recommends a frequent review of how effective doses are and any side-effects they are suffering, which typically include constipation, nausea and drowsiness.
"Until now, there has been little guidance regarding the safe prescribing of opioids," said Professor Mike Bennett, St Gemma's professor of palliative medicine at the University of Leeds and a member of Nice's Guideline Development Group.
"Despite increased availability of these medicines, pain from advanced conditions remains under-treated. This can be due to a variety of reasons such as fears over side-effects or confusion over which opioid treatment would work best.
"This guideline gives clear recommendations and should instigate a real clinical change in the way opioids are prescribed."