Macmillan Cancer Support said that 23 per cent of people who have been diagnosed with cancer feel isolated.
Patients said that their family and friends are “too busy” or live too far away, a charity spokeswoman said.
The charity, which surveyed 1,700 patients, said isolation can have a “shattering impact” on people living with cancer.
The poll showed that almost a third of isolated patients had experienced depression. Many said they had skipped meals because of lack of support and others said they were unable to perform other day-to-day chores without help.
Half of 150 health professionals also surveyed by Macmillan claimed they have had patients who decided to skip treatment altogether because they have no one to support them.
And 56 per cent felt that a lack of support at home leads to a shorter life expectancy.
Ciaran Devane, chief executive of the charity, said: “This research shows that isolation can have a truly shattering impact on people living with cancer.
“Patients are going hungry, missing medical appointments and even deciding to reject treatment altogether which could be putting their lives at risk – all because of a lack of support.
“But these figures are just the tip of the iceberg. As the number of people living with cancer is set to double from two to four million by 2030, isolation will become an increasing problem and we need to address this now.
“That’s why we are launching a new campaign to help tackle this crisis and to ensure that in future, no-one faces cancer alone.”
The charity said that healthcare professionals should ask patients whether they have support during their treatment and recovery. Health workers should also ensure isolated patients make full use of alternative support services, a spokeswoman said.