South Central Ambulance Service must provide better support and training for its staff, a health watchdog has said.
The independent Care Quality Commission (CQC) said the ambulance trust was still not meeting one of the essential standards of quality and safety.
It found that while standards had improved for 999 response times, the trust had failed to meet standards for staff training and supervision.
The ambulance trust said it took feedback very seriously.
It added it was proud of significant improvements to delays and providing high quality clinical care to patients with emergency needs.
The CQC published the report after raising concerns in April.
'Inspectors to return'
The watchdog, which grades health services across England, said the ambulance service must take action to ensure "all its staff receive an annual appraisal where their training needs can be properly assessed".
"At CQC's request, the trust has now produced plans to show how it intends to achieve compliance.
"Inspectors will return unannounced to check whether the improvements have been made and to decide whether to initiate formal enforcement action."
Last year, the CQC rated South Central Ambulance Service as "weak" for failing to meet its target response and treatment times.
It was among the three worst ambulance trusts in England after slipping from "good" in 2007/8.
The ambulance trust provides 999 cover for Hampshire, Berkshire, Oxfordshire and Buckinghamshire.