A hospital trust has been reported to the police after staff complained that they were being "pressured or bullied" to falsify data relating to cancer patients.
A number of cancer patients suffered "undue delays" at Colchester Hospital University NHS Foundation Trust, the Care Quality Commission (CQC) said.
CQC inspectors found that there were " inaccuracies" with waiting time data relating to cancer treatment.
Staff told inspectors they were "pressured or bullied" to change data relating to patients and their treatment to make it seem like people were being treated in line with national guidelines, a CQC spokesman said.
As a result some patients may not have had the treatment they needed in time, the spokesman added.
The watchdog has passed its findings to Essex Police, he said.
A force spokesman said: "Essex Police has been contacted by the CQC and is currently reviewing information it has provided to establish whether a criminal investigation is necessary."
When inspectors visited the trust in August and September they checked the national cancer waiting times system against patient records and found discrepancies in the records and types of treatment recorded for some cancer patients, the CQC said.
The spokesman said that o f 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".
In some cases people did not get their treatment within the required 62 days, and in three cases delays exceeded 100 days.
Even though an internal trust investigation in 2012 identified concerns, hospital bosses failed to investigate the allegations thoroughly or follow up with the patients who were affected, the watchdog said.
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 newly appointed Chief Inspector of Hospitals has recommended that a hospital should be put into special measures by regulator Monitor.
It is the second trust that Professor Sir Mike Richards has recommended that a trust should be put into the regime. In October, regulator Monitor put the Queen Elizabeth Hospital, Kings Lynn NHS Foundation Trust into special measures because it provided "poor care and had weak leadership".
Prof Richards said: "If you are diagnosed with cancer - you are entitled to think that your hospital will do all they can to ensure you get treatment you need as soon as possible.
"It is shocking to think that people's lives may have been put at risk for the sake of the waiting time figures.
"Every year around six thousand people go to Colchester Hospital University NHS Foundation Trust to be treated for cancer. It is essential that people in north Essex can have confidence in their hospital.
"Clearly this report raises questions over the safety and effectiveness of these services. But it also raises questions at the highest level. We have found that the concerns raised by staff in relation to changes made to people's cancer pathways were not appropriately managed or investigated by senior staff of the trust, which is why I am now recommending that this trust should be placed in special measures."
Adam Cayley, regional director for Monitor, said: "Monitor has been working closely with the CQC over its concerns in relation to this trust, and we have identified risks in the way that the trust is run.
"We have therefore opened a formal investigation into whether it has breached the conditions of its licence, and we will consider putting the trust into special measures as part of any regulatory action we may take to protect its patients.
"Meanwhile, Monitor is working with its partners to ensure the trust takes appropriate action to safeguard the health and wellbeing of all patients currently using the cancer pathway. We have also asked the trust to implement a look-back review to establish whether there is a risk that other patients did not receive treatment in accordance with national standards in recent years."
Dr Sean MacDonnell, medical director of the trust, said: " On behalf of the trust, I apologise to the patients, relatives and carers who we have let down. We are very sorry for the worry, distress and concerns that have been raised by the publication of the CQC report into our cancer services.
"Our priority is to focus on the safety and welfare of all cancer patients.
"Both myself and staff throughout the trust are shocked and dismayed by the concerns raised in the CQC report. Patients and the public can be reassured that we are taking the findings extremely seriously and are determined to get to the bottom of the issues and sort them out.
"We are learning from this inspection, we are changing the way we do things where necessary and we are doing this quickly so that we are certain that all cancer patients using our hospitals receive the best possible care.
"It is also extremely disheartening for us to learn that it appears from the CQC inspection report that some inappropriate changes were made to cancer waiting time data."
Trust chief executive Dr Gordon Coutts added: "We are very concerned by the findings of the CQC report.
"We are truly sorry that in some cases cancer patients, their carers and families have not always received the high quality of care that they should have expected from our trust. We sincerely apologise to those who we have let down and for the worry, distress and concern that the publication of today's report is likely to raise.
"We want to reassure patients and the wider public that we have already taken action in response to the findings of the report and we will continue to take action to address all of the concerns raised by the CQC.
"Our priority remains to focus on the safety and well-being of all our patients and being placed in special measures would provide us with additional advice and support to help us to further improve our services."
Christina McAnea, head of health at union Unison, said: " We cannot allow any patients, particularly those suffering from a life threatening illness like cancer, to be treated so appallingly.
"We owe it to them, to the relatives of those who have died, and to staff to make sure there is a very full and thorough investigation into the sequence of events at Colchester Hospital.
"Our members took a brave step by reporting to the CQC that they were being bullied and harassed by senior managers to falsify records relating to cancer patients.
"They raised their concerns repeatedly and in emails to senior managers, right up to the chief executive, but they were ignored.
"The NHS needs to promote an honest and open culture. Unison has been encouraging and training members to blow the whistle on any practice that might endanger patients or interfere with their treatment. I am pleased that our members in Colchester did just that."