The condition is common among elderly patients on wards and researchers claim it is both under-diagnosed and under-treated.
Research among over-70s found almost half the patients studied were suffering pain while resting, and that delirium had developed in 15 per cent.
The odds of being delirious were more than three times higher in participants in pain.
The study was jointly funded by the Alzheimer’s Society and Bupa Foundation, and supported by the terminal illness charity Marie Curie.
The study is urging hospital staff to carry out regular assessments so that pain and delirium are managed effectively.
Dr Liz Sampson of UCL said: “It’s deeply troubling to think that this vulnerable group of patients are suffering in silence, unable to tell healthcare professionals that they are in pain.
“In the UK, almost half of people admitted to hospital over the age of 70 will have dementia.
“We know they are a high-risk group for delirium and yet delirium is often under-treated.”
She added: “Our latest work suggests that pain could be a cause of delirium. Studies like this may help hospital staff provide better care now and in the future as dementia diagnosis rates continue to rise.”
Dr Doug Brown, of the Alzheimer’s Society, said the findings – published in the journal Age and Ageing – show the link between pain and delirium may be worse than previously realised.
He said: “We know people living with dementia can find it difficult to communicate. When this concerns an inability to communicate pain to hospital staff, it’s clearly extremely concerning.
“It’s not only upsetting and frustrating but can also have serious consequences on a person’s health.
“The link this research shows between delirium and pain shows that the problem may be worse than previously realised.
“We now need to take steps to ensure all healthcare professionals have the right training to identify such distress in order to properly care for people with dementia.”