The trust must improve on its response times, the report says
South Central Ambulance Service has been branded "weak" by a health watchdog for failing to meet its target response and treatment times.
The annual report from the independent Care Quality Commission grades health services across England.
The ambulance service is among the three worst ambulance trusts in England after slipping from "good" in 2007/8.
Chief executive Will Hancock said the trust was "disappointed" but aimed to bring in improvements.
The trust provides 999 cover for Hampshire, Berkshire, Oxfordshire and Buckinghamshire.
No ambulance trusts scored "excellent" in the report, two scored "good", six "fair" and three "weak".
All were marked on various areas, including safety of patients, cleanliness and waiting times.
The report said the proportion of life-threatening calls not reached by South Central Ambulance Service within eight minutes, or subsequently 19 minutes, was too high.
It also said the number of serious, but non life-threatening, calls that it failed to reach within 19 minutes was too high.
The number of heart attack patients receiving clot-busting therapies within 60 minutes was also "below a satisfactory level".
'Opportunity to improve'
Not enough strokes patients were attended to in "a timely manner" with the "correct equipment", compared with the national average.
And the number of patients with a blockage to arteries supplying the heart who received the "appropriate treatments" was below average.
However, the trust scored top marks for safety and cleanliness, dignity and respect, good management and keeping the public healthy.
The trust said it was now working to "the most exacting standards in the world".
"We have increased our numbers of front line staff. We have improved our response times to life-threatening calls by 20%.
"However despite this improvement we fell short of the targets last year."
Mr Hancock added: "We're obviously very disappointed with the results in this review by the CQC.
"But we do welcome it as an opportunity to improve the services that we provide in the future."
In total, 20 NHS Trusts were identified as being weak this year.