Unpaid carers to people with terminal cancer are providing health and social care worth £219 million each year - but health experts have warned that the Government is going to need to stump up more resources.
They said the UK's increasingly ageing population will mean there will be fewer unpaid carers able to meet the demand in future.
A study published in the journal Palliative Medicine calculated that unpaid carers account for the equivalent of a third of the cost of end-of-life care for people with breast, lung, colorectal and prostate cancers, which account for around 45% of all cancer-related deaths in England and Wales.
They warned that greater demand in future will mean there is a much larger demand for state-funded end-of-life care and could lead to more people in need missing out.
The University College London (UCL) study, described as the first to consider the equivalent costs of unpaid carers alongside both health and social care expenditure, estimated the total estimated cost (health care, social care and care from unpaid carers) for treating people with these four types of cancer at the end of life to be £641 million.
Scott Sinclair, head of policy and public affairs for England at Marie Curie, which funded the research, said it was "deeply concerning" to think how the Government would fill the £219 million gap left by carers.
"This is a very real problem," he added.
"Currently, 110,000 people a year don't get the care they need at the end of life. With fewer carers, the Government must ensure high-quality end-of-life care is available to all - in homes, care homes, hospitals and hospices.
"They must act now and discuss how to find the resources to care for increasing numbers of people with a terminal illness, regardless of what that illness is, or where an individual lives in the UK. We already know that people without a carer, partner or spouse are less likely to access the care they need at the end of their lives."
"One of our most important findings is the value of the care provided to people at the end of life by informal carers," the study concluded.
"The cost to the health and social care services to replace informal unpaid care giving with formal paid care giving would be significant. This has important policy implications. If current demographic trends continue, the proportion of older people requiring care will continue to increase, with fewer people capable of providing it.
"This also has implications for hospice services, which are highly dependent on volunteers at present. If the pool of capable informal carers and volunteers shrinks relative to the population needing care, a greater need for state funding will arise.
"Commissioners and payers of formal care services should be aware of this, as it may lead to greater pressure on resources than are currently expected."