A doctor attending pregnant asthma sufferer Tracy Kent called the London Ambulance Service at 4.25pm and asked for an ambulance to take her to casualty 'immediately'.
The vehicle arrived at her home at 5.05pm, and reached the hospital at 5.17pm.
At hospital she suffered a respiratory arrest.
Ms Kent later suffered a miscarriage. In the High Court the judge said it had never been properly explained why it had taken the ambulance so long and there was no reasonable excuse for the delay.
If it had arrived in 'reasonable' time there was a 'high probability' Ms Kent would have avoided respiratory arrest.
In appeal the NHS Litigation Authority argued that in existing law the duty of care of emergency services such as LAS - like the fire and police services - was a general one and did not extend to individual guarantees of the time of arrival.
But master of the rolls Lord Woolf ruled in appeal that the LAS was different from other emergency services - it provided 'services of the category provided by hospitals'.
It could be held liable for negligence if the facts of a particular case showed that a duty of care had been created.
In this case there had been no question of an ambulance not being available, or of a vehicle being sent to a higher-priority call.
It was foreseeable, he ruled, that Ms Kent would suffer further injuries by a delay, meaning a duty of care was created.