The proportion of ambulances hitting the target response time of eight minutes for the most urgent calls fell well below half in December.
The target for responding to Category A calls is 65% - but in December it was just 42.6%.
It was down from 51.0% in November 2014 and from 57.6% from December 2013.
The statistics were the subject of angry exchanges at Prime Ministers Questions, with David Cameron describing them as the "worst on record" and blaming "catastrophic cuts and mismanagement" of the Welsh NHS by Labour ministers.
Labour leader Ed Miliband accused Mr Cameron of having a "war on Wales" and using the Welsh NHS  for political propaganda.
Tracy Myhill, interim chief executive at the Welsh Ambulance Service, said: "We completely appreciate that this presents an unacceptable level of service delivery across the whole health and social care system."
"There are a number of mitigating factors that we need to consider.
"Firstly, we took more than 40,000 calls in December, which is a record high for us and well above what we projected for that month.
"Secondly, when you break down our 'Red' calls into Red 1s (immediately life-threatening, e.g. cardiac arrest) and Red 2s (serious but less immediately time critical, e.g. suspected stroke), we performed much better on the higher acuity Red 1 calls (57.9%) than the Red 2 calls (40.6%).
"While this is still not where we'd like performance to be, it demonstrates that we are still reaching the majority of our most critically ill patients within eight minutes."
Deputy Health Minister Vaughan Gething said the ambulance service needed to urgently improve its response to emergency 999 calls.
Leader of the Welsh Conservatives Andrew RT Davies has called for "urgent action" to address the "alarming decline" in ambulance response times.
Plaid Cymru health spokeswoman Elin Jones said paramedics were facing pressures caused by the "lack of integration of health and social care and the lack of planning".
Kirsty Williams, leader of the Welsh Liberal Democrats, said the figures were "absolutely appalling and the Welsh Labour government should be ashamed."
A Welsh government spokesperson said the figures demonstrated the immense pressure the Welsh Ambulance Service was under throughout December.
"The demand on the service was unprecedented, with the service receiving 40,147 calls during the month, or around 1,295 a day," the spokesperson said.
"They are nevertheless disappointing and there is no complacency about the need to improve performance."
The figures come as police forces across Wales said they took patients to hospital more than 100 times last month.

Kellingley Colliery is set to close in 2015 with the loss of about 700 jobs.
Workers, MPs, and union leaders marched through Knottingley and held a rally at Kellingley Social Club.
In August, Business Secretary Vince Cable said giving government funds to keep the mine open "wouldn't be good value for money".
UK Coal announced plans in April for a managed closure of the colliery due to "extremely difficult trading conditions" and in September secured Â£4m from government to support the plan.
Chris Kitchen, from the National Union of Mineworkers (NUM), said: "Kellingley has about 30 years worth of coal and we know that we're going to be burning coal in this country to generate electricity... the majority of that now is imported.
"It's not that we're trying to keep Kellingley open for some kind of historic reason, there's a use for the fuel and there's a use for the fuel going forward that's mined at Kellingley."
A report prepared for the NUM said the mine could remain open until 2018 with help additional from the government.
Labour MP for Normanton, Pontefract and Castleford Yvette Cooper said: "If the government doesn't act fast it will be too late.
"The cost of keeping Kellingley open is going up week after week because there's been no investment in new faces. Yet the government is just dragging its feet trying to kill off the pit by stealth."
A government spokesman said: "We have now received a plan from the company and will look carefully at their proposal, bearing in mind that we must make sure that taxpayers receive value for money."
Ambulance response times for the most urgent calls in Wales are the worst on record, new figures show.
[NEXT_CONCEPT]
Miners at a North Yorkshire coal mine set to close this year have staged a rally to call on the government to provide funds to keep it open.