The force was alerted at 2pm after a panel van and a scooter collided on Swallowdale Lane at the junction with Maxted Road.
The motorcyclist received serious injuries and has been taken to the Royal London hospital for further treatment. He is currently in a critical but stable condition.
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The vehicles involved were a white Volkswagen Caddy and a green Yamaha Delight (scooter).
The Bedfordshire, Cambridgeshire and Hertfordshire Road Policing Unit is investigating the incident.
Sgt Will Hood said: “We would like to speak to anyone who saw the incident today who has not yet spoken to police. In particular I’d like to speak to the driver of a dark coloured vehicle that was at the junction with Maxted Road at the time of the collision. They may be able to help us establish exactly what happened.”