An ambulance trust is being investigated after it decided to duck national targets to give itself extra time to respond to patients with potentially life-threatening conditions.
The South East Coast Ambulance Service NHS Foundation Trust ran a pilot project to delay sending ambulances until it had time to assess some calls coming through the 111 system, which replaced NHS Direct.
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 re-assess what type of advice or treatment patients needed, and whether an ambulance was really required.
The health sector regulator Monitor said the trust had not given sufficient consideration to the impact on patient safety or fully informing the trust's board.
It said the project, which was run between December 2014 and February 2015 in the south of England, 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.
Paul Streat, regional director at Monitor, said: " Over the winter, there were significant demands on the NHS and it is understandable that trusts want to explore better ways of delivering the best possible care.
"But this project was poorly managed from the start, done without the proper authorisation and without enough thought given to how it might affect patients.
"We have asked the trust to review the action it took to make sure there was no harm to patients, and look again at the way decisions are taken to prevent something like this happening again."
The trust said it had faced "unprecedented call volumes" and "serious hospital handover delays" last winter.
It said the project it undertook involved taking extra time to "re-triage" some 111 calls to 999 "to determine whether or not sending an emergency ambulance was the correct response or indeed if the case needed up-grading".
The trust's c hief executive Paul Sutton said it had wanted to make sure the most ill patients were responded to promptly, but acknowledged that it had not acted in the right way.
He said: "However we recognise that it was not well implemented and we did not use our own corporate governance processes correctly. These are serious findings.
"We have already begun to take steps to address Monitor's concerns and as part of this process, independent reviews will assess how decisions are made within the trust, governance processes and our approach to patient safety.
"As a trust, we remain extremely proud of the high quality and compassionate clinical services that SECAmb provides to our patients."
Mr Streat told BBC Surrey there was "no direct evidence that people were harmed".
But he said Monitor would go back through data to check no-one was harmed. If any were, they would be made aware of the fact.
He added: "The trust has stopped this project and they've looked at what happened, what went wrong. We need to make sure that we know exactly who made what decisions and that the right people are held to account for that and that they change the way they work so it can't happen again."
He said "this is not about forcing people to step down" but there was a need to ensure there was "no further risk in the system".
Mr Streat said if further issues came to light then Monitor had the power to change members of the leadership team.
He said patients could be assured that if they called 111 they would be "treated in the right way at the speed they should be".