A report by the Care Quality Commission (CQC) has found a number of cancer patients suffered "undue delays" at Colchester Hospital University NHS Foundation Trust.
A spokesman said that of the 61 care records examined, 22 showed that people had been "placed at risk of receiving care that was unsafe or not effective, due to delays in receiving appointments or treatment".
The CQC said that the trust has written to 30 patients, or their next of kin - where patients have died, offering to review their treatment.
The Chief Inspector of Hospitals Sir Mike Richards said "It is shocking to think that people's lives may have been put at risk for the sake of the waiting time figures".
He has recommended that the hospital should be put into special measures