Up to 20,000 patients had their ambulances delayed by an NHS trust intent on giving itself "extra time" to respond to calls, a report has found.
The review by NHS England, which will be published in full on Thursday, found South East Coast Ambulance Service NHS Foundation Trust drew up its secret plan without letting others know and failed to assess how it might affect patients.
The trust ran a pilot project to delay sending ambulances until it had time to assess some calls coming through the 111 telephone system.
National rules say 75% of Category A Red 2 calls should have an emergency response at the scene within eight minutes.
These calls are for conditions regarded as serious, such as strokes or fits, but less time critical than for calls where people are not breathing or do not have a pulse.
Under the project, the ambulance trust gave itself up to 10 extra minutes to reassess what type of advice or treatment patients needed, and whether an ambulance was really necessary.
The investigation by NHS England said the project was drawn up via a group which was established by the chief executive, Paul Sutton, and overseen by at least four executives.
But it was launched without the knowledge of 111 staff, board non-executives, the medical director or local commissioners of services.
Work is now ongoing to try to work out how many patients were harmed by the policy, including how many patients may have died as a result.
The NHS England document, reported by the Daily Telegraph and verified by officials, said 111 call handlers were assuring patients in "life-threatening" situations that an ambulance was on its way, with no idea that it was not.
Instead, paramedics with just one day's training in call-handling were ordered to phone thousands of cases back to see if ambulances were really needed.
The trust has insisted that its own investigations "have not found that the process impacted negatively on patients".
The draft report from NHS England says it is impossible to conclude that patients were not harmed, and has examined seven "serious incidents", including five deaths.
In one case - known to be the death of a 60-year-old Horsham man who had suffered a cardiac arrest - there was a "missed opportunity" to improve his outcome, it said.
The health sector regulator, Monitor, has said the trust did not give sufficient consideration to the impact on patient safety or fully informing the trust's board.
Last week, it said the project, which was run between December 2014 and February 2015, was "poorly handled" and it had concerns over how the trust was being run.
Monitor also said it has "reasonable grounds to suspect that the trust is in breach of its licence to provide NHS services", and has placed conditions on its licence.
A spokesman for NHS England South said: "We agree with Monitor's assessment that the trust acted unilaterally and inappropriately, and we support the action taken to rectify the relevant operational issues."