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.
More top news
The prominent Brexiteer says leaving the bloc will offer a "once in a lifetime" opportunity to set reform environment policies.
The Severn Crossing tolls will be abolished next year, the government has said. Currently cars are charged £6.70 for a return journey.
An Australian bride-to-be shot dead by a US police officer after dialling 911 "should not" have been killed, authorities have admitted.