There has been a sharp rise in long waits for cancer therapy in the past four years, BBC analysis shows.
The number waiting more than the 62-day target time for therapy in the past year has topped 67,000 across England, Northern Ireland and Scotland – twice as many as the same period in 2017-18.
Waits are also getting worse in Wales, but data does not go that far back.
The national cancer director for the NHS in England said staff were striving to catch up on the backlog of care.
But experts warned the problems could be putting patients at risk.
Charlotte Park is one of the cancer patients who has faced a long wait for treatment. She went to see her GP in June last year after finding a lump in her breast.
Her GP gave her an urgent referral to the local breast clinic. Ms Park, 50, from North Yorkshire, should have been seen within two weeks, but when that passed she phoned only to be told there was a backlog.
Eventually she got an appointment after three-and-a-half weeks, but only after going back to her GP to see if they could help her get seen.
She was then diagnosed with an aggressive form of breast cancer, but had to wait until September to start chemotherapy.
Fortunately, she is now in remission.
But she said: “It was so frustrating. I just felt like I was hitting my head against a brick wall. I felt under a massive amount of stress.”
“I thought I was going to die at one point. You worry so much when you are waiting. Sadly there are so many people out there is this position.”