More than 380,000 cancer patients have not been treated on time since 2015, according to data analysed by charity Cancer Research UK.
It looked at the number of patients starting treatment in England more than 62 days after being urgently referred for suspected cancer.
The NHS key target of 85% of people starting cancer treatment within 62 days has not been met since December 2015.
And despite recent improvements, Cancer Research UK said there are still too few workers and a lack of diagnostic equipment such as CT and MRI scans.
In March, the proportion of patients in England waiting no longer than 62 days to receive their first cancer treatment was just 68.7%.
Michelle Mitchell, chief executive of Cancer Research UK, said the next UK government must "make tackling cancer waits a top priority" and commit to meeting all cancer wait time targets by the end of the next parliament.
She said to do this, "all political parties should commit to a long-term, fully funded strategy to back cancer research across the UK and improve and reform cancer services in England, in order to provide our health service with much needed equipment and staff".
"Each of these numbers is a friend, family member, and loved one who is facing unbearably long waits for their treatment to begin, causing stress and anxiety," Ms Mitchell said.
'Jeopardising patients' health'
The Royal College of Radiologists (RCR) also reported delays in cancer care are becoming "routine" and the number of centres experiencing severe delays has almost doubled in a year.
Following a survey of centre bosses and clinical directors, it found 47% of units were facing weekly delays in administering treatment like chemotherapy, up from 28% a year earlier - with similar delays also reported for radiotherapy.
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Almost all (97%) of clinical directors surveyed by the RCR blamed workforce shortages for causing backlogs and delays.
The RCR found a "staggering" 30% shortfall in clinical radiologists and a 15% shortfall in clinical oncologists - figures which are projected to get worse in the next few years.
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Tap hereRCR president Dr Katharine Halliday said the reports "reveal a stark reality: the crisis in the radiology and oncology workforce is jeopardising patients' health".
She also called on the government to take action: "Time is critical. Doctors are working under extreme stress and are deeply concerned for their patients.
"We urge the new government to heed the advice of doctors and implement a forward-thinking strategy to recruit, train, and retain staff."
An NHS spokesperson said: "The NHS is seeing and treating record numbers of people for cancer, with 30% more people being treated last year than in 2015/16 and almost three million people receiving potentially lifesaving cancer checks in the last 12 months - ensuring more people than ever before have been diagnosed at an early stage and cancer survival is at an all-time high.
"It is vital that people come forward if they are concerned about cancer symptoms. Getting checked early saves lives."
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