The high number of staff off sick is affecting the ability of the Welsh Ambulance Service to reach life-threatening emergency calls.
That is the verdict by the service itself in a document seen by BBC Wales.
It also says the the service is having to incur "significant additional costs" in providing cover at overtime rates.
The Welsh Ambulance Service NHS Trust said it was reducing sickness rates year on year and has plans in place to manage absences.
Despite hitting target times for the eighth month in a row, the service's own risk assessment shows concerns about the high levels of staff absent due to long and short term sickness.
The problem was categorised as being almost certain to have "major consequences" which means it could lead to "major injury leading to long-term disability" to patients.
The service received more than 29,000 calls in September - with nearly 70% of responses to "life-threatening" calls arriving within eight minutes. The All-Wales target is 65%.
Continue reading the main storyOur sickness absence rates are reducing year on year, however we recognise more work is needed and are committed... to bring this down even further”End Quote Ambulance Service NHS Trust Recent official statistics reveal that the ambulance service has the highest sickness rates in the NHS in Wales.
As of March this year 7.3% of their staff were absent due to illness.
By the end of September that figure had reduced to 6.6% but was still higher than the average within the NHS in Wales of 5.2%.
A spokesperson for the Welsh ambulance service said: "Our sickness absence rates are reducing year on year, however we recognise more work is needed and are committed, with the support of our trade union colleagues, to bring this down even further.
"Some of the initiatives we are introducing to address this issue include setting up a health and wellbeing group, a staff support service and absence management training."
'Lot of stress'The spokesperson said they were also reviewing occupational health arrangements and sickness absence policy.
"With these plans in place, we have reduced the risk level in respect of achieving the target. This position is kept under continuous review."
Bleddyn Roberts, north Wales ambulance branch secretary of Unison said part of the reason for the sickness rates was the pressure staff were under.
"It can be very physical at times... there's a lot of stress involved as well.
"The workload has increased year on year and the number of staff within the service hasn't necessarily kept pace with that for various reasons."
