Prof Ceri Phillips: NHS caught by 'conflicting targets'
The latest statistics on the performance of the Welsh ambulance service will be published later.
It has failed to meet its response time targets for the highest priority calls for most of the past two years.
The health minister told AMs in June he expected to see "urgent improvements" within three months.
The ambulance service says its responding but BBC Wales has learnt that some improvements may not be delivered until Christmas.
Health economist Prof Ceri Phillips said there were conflicting targets in the NHS which were sometimes working against each other - which was part of the reason ambulances were waiting outside A&E departments.
He also told BBC Wales health correspondent Owain Clarke that patients with minor ailments should also be aware that they couldn't just turn up at A&E or call an ambulance and expect treatment.
24 Sep 2014