Families earning average wages with average outgoings would end up around £200 in the red each month if an adult was diagnosed with the disease, Macmillan Cancer Support said.
The charity has conducted analysis on median household income figures and average spending figures while taking into account previous estimates of the “cost of cancer”.
The average family is left with around £365 each month after paying for necessities such as bills, food and everyday travel, Macmillan said.
The charity has previously found that cancer has an average monthly cost of £570 for the vast majority of patients. This figure includes loss of income, travel to and from hospital appointments and increased household bills due to being at home more and feeling the cold after treatment.
This means that if an adult was struck down with the disease, the family could face a shortfall of around £200 a month, the charity said.
Families could be forced to take drastic measures to cover this gap including selling their homes or taking out loans, a spokeswoman said.
The charity - which offers grants, benefits advice and financial guidance for people affected by cancer - is urging patients or their families to seek financial help should they have concerns about money.
“At a time when thousands of families are struggling to make ends meet, a cancer diagnosis can be the straw that breaks the camel’s back, sending them into financial freefall,” said Macmillan Cancer Support chief executive Lynda Thomas.
“When you’re diagnosed with cancer, the last thing you need to be worrying about is how to pay the bills and keep a roof over your head.
“But the one thing I would say is that you don’t have to do this alone - there is help out there. At Macmillan we offer financial information and support to help people get back on their feet. Last year we helped 90,000 people affected by cancer unlock £267million in benefits alone.”
Single mother Devry Souayed, 55, from Lancashire, was made redundant from her civil service job a year prior to her breast cancer diagnosis in 2013. As a result of continuing treatment she is still too ill to work.
Commenting on the analysis, she said: “Cancer has stripped my health and my bank balance. I’ve had to resort to drastic measures to keep my family above water, making unthinkable daily decisions over whether to pay the mortgage or put food on the table, buy new school uniforms or to fix our leaking roof.
“The stress of living on a financial knife edge has made me physically sick. I feel so guilty that my children are missing out on school trips and I’ve had to cut back on birthdays and other treats. My elderly parents help me out financially and I have amazing support from Macmillan but three years on I’m still struggling to get by.”
Thew news comes 24 hours after the Sunday Telegraph reported that dying patients are not getting the out-of-hours care they need at the end of their lives.
Palliative care out of hours is not the “best possible care” at the moment, according to the president of the Royal College of Physicians (RCP) and other leading healthcare figures in a letter to the paper.
Even between 9am and 5pm Monday to Sunday only 37 per cent can provide access to the specialist care.