Around 160,000 men in the UK have been left with little or no sex life after treatment for prostate cancer, the Macmillan Cancer Support charity says.
It suggested rising cancer rates meant cases could more than double by 2030.
Yet many patients could be helped if more services were available on the NHS, it said.
Erectile dysfunction is a potential side-effect of surgery, radiotherapy and hormone therapy used to treat the condition.
More than 40,000 men are diagnosed with prostate cancer in the UK each year.
In some there may be permanent nerve damage, meaning they cannot maintain an erection.
In others the physical problem can be temporary while for some the treatment has led to a psychological barrier to sex.
Two in three prostate cancer patients say they are unable to get an erection.
Macmillan said men also needed to feel they could seek help when they were having problems with sex after their treatment.
Jim Andrews, 63, a prostate cancer survivor from London, said his first reaction to his diagnosis of the disease was that it would kill him.
“The thought of libido-killing drugs and sexual dysfunction still seemed minor in comparison to the alternative.
“By the time I realised I was likely to survive, my sex life had been destroyed. I was devastated.
“It was not a subject that any professional talked to me about. It’s been a lonely journey as no-one talks about it.”
Professor Jane Maher, chief medical officer of Macmillan Cancer Support, said the figures showed that this was a major problem facing patients after their treatment and not enough was being done.
“The sheer volume of men affected shows the need for careful discussions before treatment.
“Many can be helped through early intervention and better support for men living with or beyond prostate cancer,” she said.
The charity wants to see specialist nurses, better psychological support and physiotherapists more widely available to prostate cancer patients.
It says men should also be encouraged to seek help from their GP when they are having problems.
Dr Daria Bonanno, a consultant clinical psychologist funded by the charity, said: “For many men with prostate cancer there is a certain stigma attached to talking about erectile dysfunction.
“Many may feel a sense of loss of masculinity and sadness around the inability to sustain an erection and will be reluctant to seek support.
“This can often cause them to emotionally isolate themselves from their partners and could make the issues worse.”