A dramatic 160 kilometre an hour police chase in Britain took an unexpected twist when officers came face-to-face with a newborn baby in the back seat.

Four unmarked police cars boxed in the suspect vehicle on the M275 in Portsmouth after believing it had been stolen.

And they suspected the driver was doing a runner when he rushed out of the car after being pulled over.

But instead Colin Ellmore opened the back door and helped deliver his grand-daughter, Lola Mia Rose, at the side of the road.

Sergeant Simon Goss, of Hampshire police, said "We boxed it in, forcing it to stop, and when it did the driver got out and we thought he was running off".

"But he ran around the car to the passenger side and shouted something about a baby" he told the Portsmouth News.

"When we got to the car, we saw the baby in the man's hands. I have never experienced anything like it before".

Mum, Tiffany (26), described the experience last Friday as "quite scary" but said she was proud of her dad.

She realised shortly before that the baby was coming and along with her partner Steven Read, their son Ralphy (7), and her parents Colin and Elaine, the family made their way to hospital.

"As the car was moving my waters went, then the police pulled us over and my dad came round and opened the door" Tiffany said.

"I knew she (Lola) wasn't going to wait and then suddenly her head started to appear. The whole thing was quite scary".

Despite her dramatic entrance into the world Tiffany said Lola was "doing great". Police have said no action would be taken over the incident.