Two in five people in hospital never have any visitors, research suggests.
A poll of 200 hospital nurses for the Royal Voluntary Service found many believed the lack of visitors had a detrimental effect on patient recovery.
Nurses felt patients were less likely to be mobile if they had no visitors, were less likely to follow medical advice, had fewer conversations and relied on nursing staff more.
Nurses also felt these patients endured longer stays in hospital.
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Sam Ward, director of commissioned services for Royal Voluntary Service, said: "With results showing two-fifths of patients may not see a visitor during their hospital stay, it is clear that more is needed to be done to support them.
"Volunteers offer a professional support service, encouraging mental stimulation, physical activity, and can play a significant role in both mental and physical recovery.
"It is vital that hospitals work together with volunteer service providers to make sure that patients across the country are able to access this support."
Susan Webster, a senior charge nurse at Aberdeen Royal Infirmary, said: "Volunteers make a real difference to the ward and we are lucky to have such a dedicated volunteer team.
"They provide company for the patients, some of whom don't get any visitors and can be left feeling isolated.
"You can't put a price on the value of that social interaction, especially for our older patients."
The Royal Voluntary Service runs several hospital services including assistance with meals, patient transport, trolley services and volunteer-run cafes and shops.