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.