Police were called to the scene of a collision involving 97-year-old Prince Philip on Thursday. Witnesses told the BBC the Land Rover being driven by Philip had overturned after pulling out from a driveway on the A149.
Mr Warne told BBC Radio 4's Today programme: "I saw it careering, tumbling across the road and ending up on the other side.
He said that after seeing the crash, on the A149 near Sandringham: "I went to the other car. There was a baby in the back and, with another man, we got the baby out. "Then I went to the black car to help and realised it was the Duke of Edinburgh."
Mr Warne told The Sun newspaper he overheard the duke telling police he had been "dazzled by the sun".
The two women in the Kia were taken to the Queen Elizabeth Hospital in King's Lynn to be treated for the broken wrist and cuts to the knee, and were later discharged. The duke saw a doctor as a precaution.
Norfolk Police said it was standard policy to breath test drivers involved in collisions and both had provided negative readings.
The palace would not confirm if he was alone in the vehicle at the time of the collision.
Philip retired from public life in August 2017. He and the Queen have been staying at Sandringham since Christmas.