A walker has appealed for help to trace a motorcyclist who ran into him on the South Downs, leaving him injured and bleeding.

Arthur Moore, 51, was walking in the Kithurst Hill area, near Storrington, when the motorbike, being ridden illegally off-road, came up behind him, hitting him on the arm.

Mr Moore fell backwards and hit his head on the ground.

When he came round a few minutes later, he was bleeding and concussed but the biker had gone.

Mr Moore, who is a researcher at Roffey Park Institute in Horsham, said: "When I came round there was blood everywhere and I kept almost losing consciousness. I thought to myself 'If I don't get help soon I'm going to bleed to death'.

"I stumbled back to the car park and a car pulled up. A man and woman got out of the vehicle and I asked them to call an ambulance.

"I must have looked a sight -a I had a fat lip and was covered in chalk from the road as well as blood. They looked at me suspiciously and said they didn't have a mobile. I was clutching on to a nearby car because I couldn't stand properly and they walked off and left me."

Mr Moore, who lives in Storrington, said another couple helped raise the alarm after the collision at about 3pm on April 17.

The Sussex Police helicopter arrived and Mr Moore was treated for his injuries at the scene and taken to Worthing Hospital by ambulance.

He said: "I was very, very lucky.

"At first the paramedics thought my arm was broken because I couldn't move it. There was a big hole in it where it looked like the handlebar had gone in. My arm was X-rayed in hospital but by some miracle it wasn't broken."

Mr Moore, who spent five hours in hospital, had six stitches in his right arm.

Mr Moore said the incident happened on part of the South Downs Way footpath. He had been sitting in a chair near the car park reading before going for a walk.

He said: "I was miles away thinking about what I had been reading. I heard a noise behind me and turned around and saw a motorbike coming towards me. It all happened very quickly.

"I think I shouted 'Stop' and went to put up my hand. But the rider didn't brake and smashed straight into me.

"I'm not sure what happened after that. I think something hit my elbow and I fell back and hit my head.

"I lost consciousness and when I came round the bike wasn't there. The rider must have driven off."

Mr Moore, who said he often went walking on the same stretch of the South Downs, said he hoped the incident would be a warning to others.

He said: "I was angry for a couple of days but then the feeling developed into a concern other people need to watch out for these bikers.

"A lot of older people use the tracks for walking and if it had been someone elderly or a child, this whole thing could have been a murder inquiry."

Mr Moore described the rider as male and wearing dark clothing and a helmet with a black visor.

The machine is believed to have been a trail bike.

Sergeant Jon Pink, of Horsham police, said it was illegal to ride motorbikes on the South Downs.

He said: "This type of incident is very rare. We only get maybe three or four reports of motorbikes on the South Downs per year. But we are treating it very seriously.

"We would urge the rider or anyone who saw the collision to contact us."

Anyone with information should contact Sgt Pink on 0845 6070999.