An e-scooter rider aged 55 is in a critical condition with a serious head injury after he was believed to have crashed his vehicle while not wearing a helmet.
Northumbria Police said the man had been part of a group of around 12 people who were riding on hired e-scooters in Newcastle city centre on Saturday night.
It has prompted calls for riders to ensure they are always wearing a helmet when using an e-scooter, despite it not being compulsory in the UK.
A 12-month trial of e-scooters, run by Neuron, is currently taking place in certain parts of Newcastle. The vehicles can be hired using a smartphone app.
The man crashed his scooter after dropping behind the rest of his group at around 8.30pm on Redheugh Bridge Road, police said as they appealed for witnesses.
A passing driver saw the injured rider and drove ahead to flag down the people he had been with, to alert them to what had happened.
The rider was not believed to have been wearing a helmet at the time of the crash and his injuries were not consistent with being hit by a vehicle, leading police to believe no-one else was involved.
He was taken to hospital with a serious head injury where he remains in a critical but stable condition.
Sgt Matt Sykes, from the motor patrols department at Northumbria Police, said officers wished to speak to the passing motorist, who was said to have been in a Land Rover or Range Rover.
He said: "This was a very serious incident which has left a 55-year-old man with potentially life-threatening injuries.
"An investigation into the circumstances surrounding the collision is ongoing and we are keen to hear from anyone who can help us build up a clear picture of the moments which led up to this incident."
A spokesman for Neuron said: "Our thoughts go out to the injured rider and all those involved, and we wish for a speedy recovery. We are working closely with the Northumbria Police and the authorities as they conduct their investigations.
"At Neuron, rider safety is paramount. Our e-scooters are fitted with a range of safety features, including the world’s first app-controlled Helmet Lock which secures a safety helmet to every e-scooter.
"Whilst wearing a helmet is not compulsory in the UK, it is the single most important piece of safety equipment, and we strongly encourage all of our riders to wear one."