A passing motorist used a mobile telephone to alert emergency services to the accident on the A303 near Yeovil, Somerset. Officers in a passing police patrol car found the blazing vehicle on its roof.
Initial reports indicated that only one vehicle was involved in the accident on the westbound section of the dual carriageway, between the Podimore and Cartgate roundabouts.
Somerset Fire Brigade confirmed that two adults, two children and an infant were dead.
A spokeswoman for the brigade said it appeared that all the dead were from one family. A dog, believed to be a collie, also died in the blaze.
A spokesman for the West Country Ambulance Service said: "When ambulance officers arrived on the scene the car was already alight. But as they attempted a snatch rescue they were beaten back as the flames became more intense.
"The officers involved are to be offered counselling. To watch these people burn to death in their car and be unable to do anything is very traumatic."
An Avon and Somerset Police spokesman said the bodies had been taken to the mortuary at Yeovil District Hospital.
A spokesman for AA Roadwatch said the accident had shut the west-bound A303. "The A303 is always bad on a Friday afternoon and it is peak holiday time," he added.Reuse content