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A third of ambulances called to life-threatening emergencies in Kent fail to reach patients in the target time.
According to figures from NHS England, about 70% of Secamb crews reached patients within the eight minute target required for life-threatening emergency calls.
Life-threatening situations are classified as red 1 and red 2 calls.
Red 1 calls include cardiac arrest and life-threatening injuries, while red 2 refers to serious breathing difficulties and strokes.
The GMB union has blamed staff shortages for the failings, and said the government must act fast to prevent the situation getting worse.
Paul Maloney, GMB Southern regional secretary, said "The situation is at a critical level when less than 72% of life or death emergency calls are responded to within 8 minutes.
"The government has created conditions which mean the public now have to wait longer for the ambulance service to respond.
"High vacancy rates and staff shortages mean existing staff have to shoulder more responsibility and crews responding to 999 calls are not adequately staffed.
"The current leaving rate in England of 6.1% for qualified ambulance staff is outstripping the joining rate of 4.3% with the situation getting worse.
"With vacancy levels at over 1,000 in the ambulance service, the Government must act fast if they want to seriously try and hit the response time targets.
"Jeremy Hunt is acting irresponsibly, proper investment and full recognition of skills is needed to solve the problems around the recruitment and retention of ambulance staff."
The worst performing trust was East of England Ambulance Service, where only 57.6% of life threatening calls were responded to within the eight minute window
In London the figure was 63.3%, South Central Ambulance area, 72.2% and in the South East Coast area, 59.9%.
A Secamb spokesman said: “We are committed to improving our current response time performance. Year-on-year increases in demand mean we are handling more 999 calls than ever before.
"We are working hard to recruit additional staff to tackle a national shortage of paramedics and to increase the number of staff working in our control rooms.
“We are very proud of the efforts of all our staff in responding to this increase in demand.
"We will continue to prioritise responding to our most seriously ill and injured patients and would like to remind the public that they should only dial 999 in the event of a serious emergency.”