An inspection of East Midlands Ambulance Service has found improvements since the last visit but said more work needs to be done.
Inspectors from the Care Quality Commission carried out the annual routine visit for two weeks in February.
The report, published on Wednesday, identifies improvements made since the last visit in March 2013, alongside a number of areas of good practice.
But in four of the six areas investigated, further work needs to take place.
The report acknowledges that progress has been made by the service, known as Emas, in a number of areasin relation to responses to less urgent calls, infection control and deep cleaning of vehicles, staff engagement, reduction in staff sickness, and the monitoring of equipment.
Inspectors said that while there was good practice in many areas across the four non-compliant outcomes, it was not always consistent.
They said the minimum standard for response times to life-threatening calls would not be met for the full year and said the trust needs to ensure more vehicles are available at the start of a shift.
The report said the trust needs to continue with its plans to ensure more qualified staff are in the right place at the right time, and to offer better staff support.
But inspectors also noted that Emas is taking reasonable steps to progress its improvement plans.
Chief executive Sue Noyes said: “The findings of the report reflect that Emas is well-underway in delivering an organisation-wide improvement programme.
“It praises us for many areas of care and it acknowledges many improvements made since our last inspection.
“However, there are a number of areas where we know we need to improve.
“We are well underway with addressing these.”
Last week, Emas said it had provided reassurance to the bodies that oversee it that things were improving.