The critical care expertise available before a severely injured person can be admitted to hospital is "incomplete, unpredictable, and inconsistent," shows research published online in Emergency Medicine Journal.
Ambulance services are often reliant on volunteer doctors with variable levels of expertise and the availability of specialist doctors is patchy, particularly over evenings or weekends, the study shows.
This implies something of a postcode lottery of provision across the UK, and raises questions about the UK's ability to deal effectively with a critical incident involving many casualties, say the authors.
They base their findings on a survey of all 13 regional NHS ambulance services, 17 air ambulance charities, 34 organisations affiliated to the British Association for Immediate Care (BASICS), and 215 type 1 (major) emergency departments in England, Wales and Northern Ireland.
All services contacted responded to the questions on the availability and provision of critical care provided by a specially trained doctor to a seriously injured or critically ill patient before hospital admission.
All but one of the 34 BASICS organisations, which are wholly funded by charities, were only able to provide this level of specialist support on an "only when available" basis. None had a 24/7 service, and only one operated during the evenings.
Of the 327 volunteer doctors used by the 34 schemes, fewer than half (45%) said they had emergency anaesthetic skills. And while four schemes deployed critical care doctors at every incident, a third (11) said they did not deploy any critical care expertise at all.
Two thirds (11) of the 17 air ambulance services deployed a doctor on one or more of their 30 aircraft, but only five operated seven days a week. And three quarters of services relied on volunteer staff, with the availability of a doctor on any given day unpredictable.
Only one air ambulance service operated with a doctor into the night.
Just over one in four (27%) of the 215 major emergency departments had a pre-hospital care team on 24 hour standby, but only 2% had the skills to provide emergency anaesthesia. And teams with this capability had only occasionally or never been deployed.
Severely injured/critically ill patients treated by a critical care doctor before they are admitted to hospital have a better chance of surviving than those who do not receive these skills, but the cost effectiveness of deploying these specialist doctors is still a matter of debate, the authors point out.
But several reports in recent years, including from the Royal College of Surgeons, the Major Trauma Clinical Advisory Groups, and the National Confidential Enquiry into Patient Outcome and Death, have all pointed to the gaps in provision for pre-hospital critical care - and the consequences.
"The inequity is striking." Only one NHS-commissioned physician-based service exists and operates (London)," say the authors, additionally pointing to "the existence of large areas of the country in which there is no guarantee of receiving physician-based pre-hospital care at any time."
They continue: "The NHS constitution's principle of 'access to services being based on clinical need' is clearly not being applied to the critically injured before they reach hospital."
They acknowledge that regional trauma networks are being developed, but for these to work, teams led by critical care doctors working outside hospital are essential, they say.