South Central Ambulance Service (SCAS) officials say they have a “completely unacceptable” ambulance shortage.
SCAS, which serves Berkshire, Oxfordshire, Buckinghamshire and Hampshire, is often 12 to 15 ambulances short, according to a board report.
The shortage meant some crews had no ambulance when their shift started.
SCAS said it had launched a review into how many ambulances were needed, adding 52 new ambulances would be arriving in October.
The report put the shortage down to increasing 999 calls and a rise in staffing.
The report said: “We were often up to 12 to 15 ambulances short, meaning that some crews did not have ambulances at the start of their shifts.