Great Western Ambulance Service NHS Trust is failing to meet core and national standards, according to the 2006/7 Healthcare Commission report.
It was judged "weak" for its quality of service which includes items like patient safety, cleanliness and waiting times, and for financial management.
The trust was graded as "good" for meeting government targets for improving health of the population.
It was also judged to have "achieved" in reductions of MRSA infection levels.
The trust said it was "taking action to address areas of non compliance and continues to make steady progress with improvements to achieve full compliance".
There are four possible rankings for health trusts - excellent, good, fair or weak.
Great Western Ambulance Service NHS Trust is responsible for delivering a range of health services, including accident and emergency and patient transport services, to more than two million people across the Gloucestershire, Wiltshire and the former county of Avon.
The Healthcare Commission is the independent watchdog for healthcare in England.