Concerns about a "bullying culture" at a hospital now under police investigation were raised more than a year ago, it is claimed.
The Care Quality Commission has found "inaccuracies" in the cancer waiting time data at Colchester Hospital.
The Royal College of Nursing says the hospital dismissed its concerns raised more than a year ago as "fantasy".
The hospital said an investigation carried out by its own management in February 2012 was "not adequate".
Staff told inspectors they were "pressured or bullied" to change data relating to patients and their treatment to make it seem people were being treated in line with national guidelines, the Care Quality Commission (CQC) said.
The inspector said patients' lives may have been put at risk.
Essex Police has been sent information by the CQC and is currently investigating whether there are grounds for a formal criminal investigation.
Karen Webb, the regional director for the eastern region at the Royal College of Nursing (RCN), said she raised concerns a year ago about the "bullying culture, the secrecy of the management culture" at Colchester Hospital University NHS Foundation Trust.
She said RCN members who voiced these concerns were dismissed as "fantasists" and the board of the hospital tried to bully them into "shutting up".
"I think there are a few things that smell rather unpleasant about Colchester General and the way that it is being led," Ms Webb said.
"It is a great shame the trust chose not to listen to us over a year ago. Not only has it not acted on it, it behaved in the way that bullies classically behave.
"Regularly, staff at Colchester Hospital have been told to keep quiet, to shut up, to not say anything about the directions that they've been given by senior managers to make sure that the hospital meets its target by massaging the data."
Responding to her claims, Dr Gordon Coutts, the hospital's chief executive since July 2010, said there had been an investigation into the allegations in 2012 carried out by two members of the management team.
This probe, he said, "was not adequate" and "did not go deep enough".
Asked by BBC Essex if he would resign, Dr Coutts said: "My job is to keep improving this hospital, and I intend to continue to do so."
Tim Roberts, the regional co-ordinator of Unison, said it first raised concerns last year.
"Our members said they were being bullied, they were under undue pressure by middle and senior managers to falsify records, to put erroneous data on the cancer pathway database," he said.
"The dates when a patient was referred to hospital - they were asked to change the dates.
"The result was the trust would look like it kept to the cancer pathway deadline set by the Department of Health.
"There's a high likelihood patients received treatment later because the wrong date was put on the database."
Colchester MP Sir Bob Russell said he was "appalled" at the findings that showed staff falsified patient data.
Sir Bob said what has happened "cannot be justified under any circumstances".
"What level was this knowledge known? Senior people will have to answer for what they knew," said Sir Bob.
"The consequences may well be that some people in senior positions may go or be asked to go."
NHS England said it would now lead an incident management team of cancer specialists, to ensure the safety of cancer patients at the hospital.
There will also be a review looking back to as far as 2010 to check whether any other patients had their notes changed or "inappropriately recorded".
Professor Sir Mike Richards, chief inspector of hospitals, recommended Colchester Hospital University NHS Foundation Trust be put into special measures, which could lead to a new management team being installed or another trust put in charge.
The hospital has set up a helpline for any patients or families who have concerns - 0800 028 2026.