Two men were injured and three horses died after a trailer carrying 10 of the animals overturned on the A303 eastbound carriageway, after colliding with a lorry.
Seven were sedated and removed by vets and expert animal rescuers but one was killed in the crash and two had to be put down.
Hampshire Fire and Rescue Service said the drivers of both vehicles were not seriously hurt.
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A number of teenagers, aged 13 to 15, from Portsmouth, have been treated in hospital, after taking tablets believed to be diazepam.
Thursday Evening's Pollen Forecast
Thursday night's weather for the east of the region