An ambulance trust has been asked to check whether any patients were harmed after it gave itself 10 extra minutes to deal with some life-threatening calls.
The three-month project run by South East Coast Ambulance Service NHS Foundation Trust during a busy winter period was "poorly handled", the health regulator said.
Monitor has ordered a review of the programme, saying it "has reasonable grounds" to suspect the trust is in breach of its licence.
The trust changed how it handled some NHS 111 calls which were transferred to the 999 emergency system to give itself extra time to deal with more urgent calls, Monitor said.
The majority of these calls, in the second most serious category and classed as life-threatening but less time-critical, are expected to be dealt with within eight minutes under national standards.
Between December last year and February, the trust allowed itself up to 10 minutes more to re-assess some calls and decide whether an ambulance was needed.
The project had not been properly authorised and not enough thought was given to the impact the change might have on patients, Monitor said.
The trust's leadership team could be replaced if there is not enough progress made following the review, it added.
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."