In England, nearly 17,000 patients were made to wait longer
than the recommended time limit for undergoing vital diagnostic tests for
cancer and other diseases, the latest figures show.
The figures also show an extra 250,000 patients are waiting for planned surgery or treatment compared to last year.
This long delay puts the patients' best chances of survival at risk.
They blamed staff shortage and lack of enough MRI and CT machines for the delay.
The NHS constitution recommends no more than six weeks' time for a diagnostic test to take place from the time of referral by their general practitioner (GP).
But, the ratio of people not being able to get their tests done on time has doubled from 1.1% last year to 2.2%, which includes potentially some with cancer.
Labour's shadow health secretary Andy Burnham said "Patients are waiting longer for crucial tests - causing stress and real anguish for worried families.
"Two weeks ago, the NHS missed the cancer treatment target for the first time ever and these delays have clearly played a part in that.
"All the progress made on cancer care in the last decade is now at risk."
The proportion is also the highest in six years indicating the poor or no progress made in recent years to tackle the problem.
Ciaran Devane, of Macmillan Cancer Support, said "Each individual hospital has a responsibility to meet these targets, or they risk putting a patient's best chance of survival at risk.
"However, this government and the next also need to take responsibility. Macmillan Cancer Support is urging all political parties to make cancer a top priority at the upcoming general election."
The Department of Health has shelled out an extra Ģ250million to hospitals and GP-led organizations to bring down the waiting times.
According to the department, the increase was, in part, due to the ageing population, which indicates more patients are being referred by GPs.