Hampshire Police said a man in his 20s was arrested at the scene of the crash on suspicion of causing death by driving without due care and attention.
The double-crewed ambulance belonged to South Central Ambulance Service NHS Foundation Trust (SCAS) and was en route to Southampton General Hospital with the elderly patient.
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Ian Paterson was convicted of 17 counts of wounding with intent and three counts of unlawful wounding.
The child suffered months of headaches that left doctors baffled. He was finally diagnosed after he drew an Etch-A-Sketch of his condition.
A guarantee from City Hall to bankroll the annual £2-3m bill was a key planning condition imposed by Westminster and Lambeth councils.