Around a quarter of cancer patients experienced an "avoidable delay" in their diagnosis over one year, according to a study by scientists from Cambridge.
Delays in seeking help, long waiting times for tests, a shortage of diagnostic staff in hospitals, and difficulty interpreting vague symptoms are some of the reasons behind the waits, Cancer Research UK (CRUK) said.
A study led by the charity looked at data from the national cancer registry for 2014 on 14,259 people diagnosed with cancer in England.
Changes now allow for earlier investigation and referral of patients with symptoms which could be cancer-related.
The researchers found that 3,372 patients (24%) experienced an avoidable delay. Half of these patients waited around two months longer for their diagnosis compared with those who did not experience a delay.
Thirteen percent of the avoidable delays happened before the patient saw their GP, 38% after being referred to hospital and almost half (49%) while the patient was being assessed by their GP.
This is in part due to waiting for tests to be done and for their results to come through.
Overall, across surgeries and hospitals, long waiting times for tests were responsible for a quarter of all avoidable delays, which CRUK said is an indication of shortages in diagnostic hospital staff.
Other reasons for the delays include patients having vague symptoms which may not be immediately linked to cancer, and/or having pre-existing conditions that make it harder to differentiate symptoms.<
Ruth Swann, lead author and Cancer Research UK's senior cancer information analyst, said: "Understanding when patients are experiencing delays leading up to their cancer diagnosis is really important to ensure people aren't waiting unnecessarily for a potential diagnosis.
"Our research shows there's a good opportunity to significantly reduce delays by cutting the time it takes for patients to have tests done. We need more research to develop and evaluate new diagnostic tests for patients with vague symptoms and a better way to manage them."
Sara Hiom, Cancer Research UK's director of early diagnosis, said there "simply aren't enough people in the NHS to read scans or report tests swiftly".
The charity said that while more patients with suspected cancer symptoms are being referred earlier, hospitals do not always have the staff to meet this growing number.
The number of people diagnosed with cancer each year in the UK is set to rise to more than 500,000 by 2035 - one person every minute.
Pre-consultation avoidable delays were more likely among breast cancer patients, while delays at GP-level were more likely in patients with multiple myeloma and least likely in patients with breast, endometrial and renal cancer, the study found.
Avoidable delays at hospital level were more common in patients with prostate, colon, lymphoma, pancreatic, endometrial and renal cancers
An NHS spokeswoman said: "This report is based on data from 2014 and does not reflect significant improvements in care and treatment, such as a lowering in the threshold at which GPs refer patients for a cancer test, that has happened in the five years since.
"The NHS carried out 2.2 million checks last year, the most ever, and research released just last week shows that cancer survival is at a record high."
Professor Martin Marshall, chairman of the Royal College of GPs, said almost 80% of all cancer cases are now referred after no more than two consultations.
"GPs have to balance the risk of not referring a patient with that of over-referring, which can cause unnecessary concern for patients and risks
overloading specialist services," he said.
"It's a decision that is exacerbated, as many symptoms of cancer are vague and often likely to be other, more common conditions."