Ambulances in three regions of England are "failing" to get to some of the most critical patients on time, new figures suggest.
Paramedics should strive to get to at least three quarters of the most serious cases within eight minutes, but three of 11 ambulances services across the country are not meeting this standard, according to the Health and Social Care Information Centre (HSCIC).
Latest data show that ambulances services in the East Midlands, the East of England and the South West are failing to get to 75% of "red one calls" within eight minutes.
These calls are classed as such because they are the most time critical - they include cardiac arrest patients who are not breathing and do not have a pulse, and other severe conditions.
In 2013/14, the East Midlands Ambulance Service NHS Trust reached 71.3% of these patients within eight minutes, South Western Ambulance Service NHS Foundation Trust reached 73.1% in the time frame and the East of England Ambulance Service NHS Trust reached 73.6%.
Across the country the average response rate within eight minutes for red one calls was 75.6%.
The HSCIC said: "The national agreed standard for Category A calls is for emergency response vehicles to arrive on the scene within eight minutes in 75% of cases.
"Of the 11 ambulance trusts, three failed to achieve the eight minute response time for Red one calls."
A spokesman for NHS England said: "These figures show that NHS ambulance services continued to perform well overall, but in some areas there is work to be done to make sure people get a consistent, high-quality service no matter where they live.
"It is important to stress that the majority of patients are treated quickly and effectively despite demands on ambulance services over the last year.
"But it is quite clear that ambulance trusts are under pressure and we will need to address this when allocating the extra money recently announced to help the NHS meet the high standards that patients are entitled to expect."
A spokeswoman for the East of England trust said: "The trust appointed new chief executive Anthony Marsh in January, who put in place a performance improvement action plan as the trust has been consistently failing its targets and performance.
"Fundamentally this is due to not having enough staff or ambulances on the road. Therefore the trust is currently recruiting 400 student paramedics this year. So far it has already offered 360 contracts to student paramedics and more than 120 of these have started their training or are on the road.
"In addition, the trust has brought in 147 new ambulances and is upskilling existing staff from emergency care assistant to emergency medical technician and EMT to paramedic."
A South Western spokesman added: "The Red 1 performance standard is a challenging one especially as South Western Ambulance Service Foundation Trust is the most rural ambulance service in the country and there is a direct correlation between rurality and performance.
"While we only reached 73.1% of Red 1 calls in eight minutes in 2013-14, we did arrive on scene within eight minutes and 17 seconds 75% of the time."
Sue Noyes, chief executive of East Midlands Ambulance Service NHS Trust said: "We are naturally disappointed that we were unable to achieve our performance targets for 2013/2014.
"Towards the end of 2013, we introduced our Better Patient Care plan. This heralded a series of key changes all of which were focused on improving EMAS' overall performance."