It can take up to a year for some bowel cancer patients in the UK to start treatment, according to international research co-ordinated by Cancer Research UK and published in BMJ Open.
Researchers found that it took a year or more from first spotting a symptom to beginning treatment for 10% of patients in England, Northern Ireland and Wales.
A team from the International Cancer Benchmarking Partnership (ICBP) - a collaboration between countries with similar healthcare systems and high-quality data - tracked each step people with bowel cancer went through before treatment. They examined questionnaires, completed by 2,866 international patients and their doctors, as well as medical records of patients diagnosed between 2013 and 2015.
They found that men and women in Wales took the longest to contact their doctor once they had noticed a health concern or symptom (49 days on average).
Once cancer had been diagnosed patients in Wales then waited the longest (39 days on average) before starting chemotherapy, radiotherapy or having surgery - more than double the length of time for patients in Denmark and Victoria, Australia (14 days).
Overall, patients in Wales had the longest time than any of the other areas in the study between noticing a change and beginning treatment (168 days on average).
This compared to 145 days in England, 138 days in Northern Ireland and 120 days in Scotland. Denmark performed the best with the process taking 77 days on average.
Between the countries there was also a big difference in the time it took for patients to be diagnosed, once they had sought help.
Patients in Denmark and Victoria most commonly waited 27 and 28 days respectively to receive a bowel cancer diagnosis, compared to patients in Manitoba in Canada who waited 76 days.
In the UK, patients in Wales and Northern Ireland waited longer to receive their diagnosis (on average 60 and 64 days respectively) than patients in Scotland (38 days) and England (48 days).
By comparing healthcare systems in similar countries, the ICBP can help identify important differences to inspire improvements in diagnosing cancer across the world and help save more lives.
Waiting longer to begin treatment can increase patient anxiety and may also impact on the success of treatment.
Sara Hiom, Cancer Research UK's director of early diagnosis, said: "This work shows that the UK has a major task ahead to improve how promptly bowel cancer patients receive treatment.
"There is much we can learn from other countries, from addressing barriers to encourage people to visit the GP if they notice unusual changes to ensuring they have the swiftest possible path from referral to diagnosis and treatment.
"It's also essential we have enough staff and capacity to perform and report tests promptly and deliver treatment without delay. Diagnosing bowel cancer relies on trained endoscopists and pathologists and there have long been shortages of these vital health professionals across the UK.
"Increasing the necessary workforce so that patients can get the care they need, when they need it will ensure a less stressful and worrying time for patients and their families, as well as extending lives and ultimately saving NHS spending on costly treatments. Without this investment in the NHS, the Prime Minister's commendable ambition to improve early diagnosis will not be realized."
Professor David Weller, lead author based at the University of Edinburgh, said: "The significant variation between participating countries in the time it took for patients to begin treatment shows there is real potential to speed up this process.
"Further research is needed to understand these differences and build on what is working well in other countries to give patients in the UK the best possible care."