The UK has around 110,000 "sandwich generation" carers who are looking after a parent with cancer and their own children, figures suggest.
New data from Macmillan Cancer Support estimates that almost one in 10 cancer carers are "sandwich carers", and most of them (89%) are also juggling a job.
A new report from the charity, based on research among almost 900 cancer carers, found many suffer mental health problems as a result of caring, such as feelings of anxiety and depression.
Almost one in three carers (30%) said their income or household finances are affected by caring, while 43% of those in work say caring affects their working life.
Cancer carers are now spending an average of 17.5 hours a week looking after someone with cancer, 2.5 hours more than in 2011, the report said.
Compared with five years ago, cancer carers are also more likely to be over 45, caring for someone with a terminal diagnosis and are more likely to provide hands-on and complex care.
Almost four out of 10 (38%) now help with healthcare tasks such as giving medication or changing dressings, up from 28% in 2011. Just over a third (34%) help with washing, dressing and going to the toilet, up from 23% in 2011.
But the report found that 55% of carers are not getting any extra support to help them with their caring responsibilities.
The overall number of cancer carers has risen by almost a third (31%) to almost 1.5 million in the last five years.
Lynda Thomas, chief executive of Macmillan Cancer Support, said: "It's saddening to hear of the growing strain on cancer carers.
"In extreme cases a person may have to dress, feed and take their parent to the toilet whilst also dealing with the school run and a full-time job.
"Too often, this "sandwich generation" of carers find themselves pulled in every direction by a physically and emotionally draining juggling act that can cause their finances to come under pressure, their working lives to suffer and their own health to bear the brunt.
"It's not just "sandwich carers" that are facing this uphill battle. Carers across the UK, looking after their mothers, sisters, brothers or friends, are carrying out more caring tasks and for longer.
"Many are doing it with a real sense of pride and privilege but this doesn't mean it isn't difficult."
Ms Thomas urged anyone who is struggling to get in touch with the charity.
Nikki Crossley, 41, from Kent, discovered she was pregnant with her second child in 2014, a month after she found out her mum's breast cancer had spread to her brain.
She said: "Becoming my mum's carer just kind of happened. Before I knew it I was in charge of her medication, her doctor's appointments, cooking her meals, paying her bills, helping her dress and get to the toilet.
"Recently, she became confused after a bladder infection and I was up half the night trying to settle her, then I still had to get up early to sort the kids out and head to work myself, exhausted."