When a 999 call requesting an ambulance is received, the operator who initially answers the call will allocate the call a code based on the conversation they have with the caller and their interpretation of the information they have been given.

There are 6 codes, Red 1 and 2, which are for potentially life-threatening cases, and Green 1, 2, 3 and 4 for less severe calls. All ambulance services in England prioritise red code calls based on nationally agreed targets and green code calls on recommended response times for each level of code.

Allocation of codes does, to a certain extent, rely on the individual taking the 999 call and mistakes unfortunately have occurred with calls being incorrectly categorised. However, there has always been a very strong reluctance in English law to hold emergency service providers liable to negligence. It has, in the past, been held that the Fire Brigade, Police and Coastguard etc. have no duty to go to the assistance of those in peril, or whose property is in peril. However, in 2000, the Court of Appeal, in the case of Kent v Griffiths, held that an unreasonably delayed response by the ambulance service to an emergency could be actionable negligence. Despite this ruling, there is not inconsiderable practical difficulty proving a breach of duty when faced with a delay in an ambulance arriving.

Another type of transportation error that may sometimes occur is when the ambulance does not immediately transport the patient to a hospital equipped to deal with the condition that the patient is suffering from. This delay in the patient attending the correct hospital may mean that the outcome for the patient is worse than it might have been and a potential claim may therefore arise.