The South Central Ambulance Service spokesman said: "There was a collision towards the front of the queue and the more serious one about half-way down."
He added three ambulances, three rapid response vehicles and two ambulance officers and an air ambulance car had attended the scene.
Six vehicles in the middle were said to be badly damaged, while a further 10 to 15 were also involved.
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Another unsettled day tomorrow with the best sunshine stretching diagonally from northwest Scotland to southeast England.
Jack Letts has learned he has been stripped of his UK citizenship in an exclusive interview with ITV News - and says it is not a surprise.
Risk of thunderstorms across East Anglia and the South East this afternoon. Becoming drier across Wales and the West Country by late pm.