Cancer Research UK says up to 1,900 Covid-19 tests are needed a day in Wales to get cancer services back on track.
The charity's experts have also calculated a drop of almost 14,000 urgent GP referrals for suspected cancer in Wales since lockdown began ten weeks ago.
It says protecting diagnosis and treatment areas from the virus must be an urgent priority for cancer services to become fully operational again.
In Wales, the charity says this would mean scaling up the nation's testing programme by between 1,000 and 1,900 tests a day.
Analysis by Cancer Research UK found that testing, together with enhanced infection control procedures and an adequate supply of PPE, will help protect patients and staff from Covid-19.
The findings are released as Cancer Research UK launches a campaign highlighting the importance of Covid-19 testing to ensure all cancer services operate safely.
Andy Glyde, Cancer Research UK's public affairs manager in Wales, said: "It's deeply concerning that patients are not receiving the cancer diagnostic tests and treatment they need.
"This is partly due to people who have worrying symptoms not visiting their doctor because they're frightened, they'll become vulnerable to the virus when they go for tests in hospital.
"Cancer testing and some treatment has also significantly stalled due to the pandemic, putting lives at risk.
"To address this, we need to see COVID-19 testing efforts ramp up for staff and cancer patients so everyone has confidence that diagnosis and treatment areas are safe.
"In some cases, this would lead to lives being saved. Early diagnosis can significantly improve someone's chances of survival. It's crucial patients don't wait for the pandemic to be over before they receive the treatment they need.
"This is why it's essential we see all cancer services fully operational in Wales. This means taking action to quickly ramp up the number of virus tests offered."
We are working with health boards to resume more essential NHS services for people, including cancer care and screening services, while continuing to protect the health service’s ability to respond to future peaks of coronavirus. Throughout the pandemic, the NHS has continued to provide urgent cancer care and treatment. We would urge anyone who is worried about cancer or symptoms not to delay seeking help - the NHS is there to help people who need it.