Only 46.5% of emergency ambulances arrived on time in Caerphilly throughout December 2012, according to Welsh Government figures.
Caerphilly County Borough had the third worst rate in Wales.
The target response time for emergency ambulances is within eight minutes.
Across Wales the response rate was the lowest in 2012 with the 65% all-Wales target missed for the seventh successive month.
Only 56.1% of calls in December were responded to within eight minutes.
Kirsty Williams, Leader of the Welsh Lib Dems,
Kirsty Williams, leader of the Welsh Liberal Democrats, said: “People in Caerphilly are not getting the service they deserve at critical times in their lives.
“One major reason for these dismal figures is the persistent failure of the Welsh Labour Ministers to establish what resources are needed to match rising demand for ambulance services.
“An ambulance service that is able to respond quickly, with well-skilled, motivated staff is often the crucial first stage in a response to medical emergencies. While paramedics and technical staff are striving for excellence, they are facing immense pressure and having to work with increasingly stretched resources.
“Until now, Wales has funded its ambulance service to meet a target of 65% of ambulance responses to life-threatening conditions within eight minutes – whereas England funds to achieve 75%. The Minister herself should now decide to raise that target, so that the review can be based on realistic funding to attain it.
“What Wales does not need is another period of re-organisation, such as by breaking up the all-Wales ambulance service and losing many of its specialist personnel. Uncertainty may cause some to go in the next few months unless it is made clear that the review will be focused on financing and efficiency – not on breaking up the organisation.”
A spokesman for the Welsh Ambulance Service said: “During the month of December 2012 we dealt with 38,135 emergency incidents across Wales, an increase of almost 12% on November’s figure, 34,168.
“In recent months, the healthcare system across Wales has faced significant pressure with increased cases of winter-related illnesses, such as flu and sickness viruses, and the ambulance service in general continues to experience a higher volume of calls.
“We would like to reassure the public that the trust is firmly committed to delivering the improvement aims of its Working Together for Success modernisation programme and will continue to work with health boards and other partners to improve our services for the people of Wales.”