Police have launched a major investigation after officers in York admitted they may have lost live ammunition on the streets of the city.

A magazine containing 15 9mm machine pistol rounds may have been accidentally left on the roof of an armed response vehicle (ARV) as it drove from the Fulford Road police headquarters into the city centre. Despite an extensive search, the ammunition - which is live and dangerous and should not be touched - has not been found.

An investigation has been launched into the incident, said to be first of its kind involving North Yorkshire Police.

The magazine went missing after an officer with the ARV, a marked Volvo estate car, removed it to demonstrate a weapon to a party visiting Fulford Road on Monday evening.

The visitors, who were not police officers, had asked to see equipment in the ARV and the officer took the magazine from his Heckler and Koch MP5 machine pistol - a weapon favoured by military special forces around the world - so he could safely allow them to examine it. But North Yorkshire Police spokesman Tony Lidgate said that was the last time the magazine was definitely seen.

"The ARV turned right into Fulford Road and was driven into the city centre. The loss of the magazine was discovered on Wednesday morning.

"The car has been searched without success. One of the possibilities is that the magazine was put on the roof and was still there when it drove off. Officers have combed the station yard and made detailed on-foot searches along the road as far as St George's roundabout, the last point at which it's thought the object would have stayed on the car roof."

Assistant chief constable Peter Walker said: "Live ammunition is not a plaything, it is very dangerous."

He said if the rounds were dropped or hit, they could fire.

He urged anyone who had found the ammunition to call the police immediately and not touch it.

The magazine is made of black matt metal, rectangular in shape, and six inches long. The bullets are brass-coloured and an inch long.

Mr Walker said: "This is the first such incident in the history of North Yorkshire Police, and we take it very seriously indeed. A stringent investigation is already under way, and we shall make it a top priority to examine procedures with a view to making a reoccurrence impossible."

Chief Inspector Ron Johnson said the Police Complaints Authority had declined to oversee the investigation, which would be carried out by the force's own Professional Standards Department.

He could not speculate on its findings, but if it recommended disciplinary action that was likely to be acted on. No officer has been suspended.

Police do not believe there was anything sinister about the disappearance of the ammunition.

Today, Fulford Road residents expressed shock and concern.

Julie Simpson, a mother-of-one, said: "You would hope that the police would always take such care about this sort of thing. It should have never happened that live ammunition has been lost."