The charity is calling for investment in diagnostic staff and equipment in order to tackle the backlog.
Last year, an NHS review of diagnostic services said an extra 4,000 radiographers and 2,000 radiologists were needed in the next five years.
Ms Mitchell said: “Despite the Government’s commitments to tackling the NHS backlog, the number of people waiting for diagnostic tests continues to grow.
“In the upcoming Comprehensive Spending Review, the Government must provide the NHS with investment for more staff and equipment to tackle the unacceptable backlog of people waiting for tests, bring down waiting times and get people diagnosed more quickly.
“Without urgent action from the Government, I’m worried that cancer survival in this country could go backwards.”
Last month, a study by the Institute of Public Policy Research estimated that 20,000 cancer diagnoses had been missed during the pandemic. It warned that clearing the backlog of patients waiting for treatment could take more than a decade and come too late for thousands.
A Department of Health and Social Care spokesman said: “We are ensuring the NHS has everything it needs to continue providing quality care to the public as we tackle the backlog. Nearly half a million people were checked for cancer in June and July – some of the highest numbers ever.
“We have provided record investment to tackle the backlog, including £2 billion this year and £8 billion over the next three years, which will deliver an extra nine million checks, scans, and operations for patients across the country.
“We have also invested a further £350 million to set up 40 new Community Diagnostic Centres which will provide around 2.8 million scans in the first full year of operation.”