An air ambulance has landed at the scene of the crash on the M25 near Watford where a man and woman have suffered serious injuries.
The accident involves a Citroen C3, a Nissan Qashqai and a lorry near to junction 20 of the M25 clockwise, near Kings Langley.
The occupants of the Citroen, an elderly man and woman, have suffered serious injuries.
They have been taken to hospital for treatment.
Officers remain at the scene and the road will be closed for a number of hours to allow a full investigation to be carried out.
They are working to clear some of the congested traffic and motorists are advised to avoid the area.
Traffic is being diverted via the A41 and A405, to leave at junction 19 and rejoin the M25 at junction 21a, and Highways England is estimating an hour delay for drivers.
Witnesses to the collision who have not yet spoken to police officers should call 101 with reference ISR 290 of March 27.
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