Emergency services are at the scene of a two-car crash on the A4130 in Remenham.
Traffic is being diverted and motorists are being advised to avoid the area for the next hour.
Two vehicles are involved in the collision, a silver Skoda Octavia travelling away from Henley and a silver Vauxhall Zafira, travelling towards Henley.
The driver of the Skoda, a man, has been taken to hospital with serious injuries.
The occupants of the Vauxhall, two adults and two children have also been taken to hospital where one of the children, a three-year-old girl, remains in a critical condition.
PC Mark Brazier said: “I appeal to anyone who saw the collision, or the cars being driven beforehand, to contact me.”
Anyone who may have witnessed the collision is urged to contact PC Brazier via 0845 8 505 505, or if you don’t want to talk to the police or give your name, call the Crimestoppers charity on 0800 555 111.
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