Scientists looked at data from across the globe and found that cancer patients who contracted COVID-19 face a mortality rate of 22.4%.
This is 273 time higher than the mortality rate for COVID-19 sufferers who do not have cancer, the Daily Mail reported.
The study, which was published in the Journal of the National Cancer Institute, also shows that those with lung and blood cancer face a higher risk.
The survival rate for those with lung cancer and COVID-19 is 67.1% while those with blood cancer and COVID-19 have a 65.8% chance of survival.
Researchers looked at 15 studies from across the world which in total involved 3,019 patients and also found that treatment such as chemotherapy has no effect on mortality rate.
Professor Justin Stebbing, study lead and professor of medicine and oncology at Imperial College London, told the Sunday Telegraph the findings were surprising.
He said: 'One in five is the death rate in COVID in cancer, 22.4 per cent, that is a really, really high number. We must take heed.
'The NHS is open, and will carry on with cancer treatment, but the lesson from these findings is that we have to prioritise these very vulnerable people.
'A lockdown doesn't apply to hospitals but it does tend to cause that as an aside.
'Prioritising virus testing for patients and their care workers, and including these patients in the early roll-out phase of vaccines, is what we must now do.'
The authors of the study said patients should be encouraged to attend routine cancer screenings and follow ups as long as 'proper precautionary measures' are taken prior to a clinical visit.
As well as increased risk for those with lung and blood cancer, the research also found that being aged over 65 and being male also increased the risk of mortality.
Other known risk factors such as diabetes and heart disease were less significant to those in the cancer group.
The alarming data comes as a charity warned the country was at a crossroads with cancer care and urged the Government to protect it during the second lockdown.
Cancer charity Macmillan warned that as many as 50,000 people in the UK have cancer which has not yet been diagnosed because of the disruption caused by COVID-19.
Lynda Thomas, Chief Executive at Macmillan Cancer Support, said: 'Cancer care is at a crossroads and services cannot be shut down this winter.
'Because of the pandemic, we estimate that an additional 50,000 people are missing a cancer diagnosis and others are having their appointments disrupted once again.
'It is simply unacceptable if they face unbearable and unprecedented delays which could affect their chances of survival.
'Cancer doesn't stop for COVID-19 and neither can our health services. Macmillan is doing whatever it takes to support people with cancer and our exhausted NHS staff, but we need more.
'Governments need to promise every person with cancer that they won't be forgotten and ensure cancer services are protected, come what may.'