Three men believed to be in their late teens and early 20s have been killed after their car and an articulated lorry collided.
A third person was taken to hospital with serious injuries following the crash on the A10 near Turnford, a spokesman for Hertfordshire Police said.
The accident happened on the northbound carriageway at around 2.15pm when a Vauxhall Corsa collided with the Mercedes Artic lorry.
The driver and two passengers in the Corsa died at the scene while a third passenger was taken to the Royal London Hospital, Whitechapel, by air ambulance with serious injuries.
The driver of the lorry was not injured.
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