Horrified drivers on the M25 were forced to swerve out of the way after two dogs were thrown from a car into 60 mph traffic.
The helpless animals were seen being hurled out of the vehicle onto the busy motorway on Sunday evening at around 6.30pm.
One of the dogs, a lurcher, was killed instantly after being hit multiple times by passing vehicles.
The second, a jack Russell terrier, was able to dodge the traffic and flee to the side of the road.
Drivers pulled over to try and help him and one caught the dog and waited for RSPCA officers to arrive at the scene.
The pooch, nicknamed Tarmax by animal welfare officers, was rushed to the vets is not believed to have suffered lasting injuries.
RSPCA animal welfare officer Peter Warne said the incident was “absolutely outrageous”.
“The danger that this act put not only the dogs in, but also the many people traveling on the road at that time, is just appalling,” he said.
He said that the incident happened between junction 19 and junction 20 of the motorway but no one was able to give a clear description of the vehicle the animals were thrown out of.
“It is terribly sad the lurcher dog did not survive and was killed in such a horrific way - it seems this poor dog’s death was instant due to the speed that the vehicles were travelling at,” he added.
“I’d like to say big thank-you to those drivers who were able to safely slow down and help to catch the dog, and the person who waited with him until I arrived. Those people almost certainly saved this dog’s life.”
Tarmax, believed to be between six and seven years old, has been moved to a temporary foster home to be looked after.
Anybody with any information can call the RSPCA’s inspectorate appeal line number, in complete confidence, on 0300 123 8018.