Armed police officers protecting the London mayor, Sadiq Khan, allegedly left a bag containing guns and a Taser on the street which was discovered by a member of the public.

The Metropolitan police said on Friday it was investigating the incident and five officers had been removed from frontline duties while inquiries were being carried out.

The bag was found on a kerbside in south London at about 9.40pm on Tuesday. A Met spokesperson said: “We have made a referral to the Met’s directorate of professional standards following an incident which took place on Tuesday 31 March.

“At around 21:40hrs, a member of the public called police after finding a bag containing Met-issued firearms and a Taser on a street in south London.

“Within seven minutes of the call to police, officers arrived at the scene and safely recovered the items.

“We are urgently reviewing the circumstances of this incident and recognise the concern it may cause. At this stage it is believed the bag was misplaced by on-duty officers a short time before the member of the public located it.”

The bag contained an MP5 semi-automatic Heckler & Koch carbine, a Glock pistol, Taser and ammunition, the Sun reported.

Jordan Griffiths, a scaffolder, said he called the police after his pregnant partner saw the bag.

He told the Sun: “My partner saw the bag sitting in the road by the kerbside and became suspicious. She went over to it and kicked it to see what it might be. But it was really heavy and didn’t move.

“It was lucky one of the guns inside didn’t go off or else she and our baby, due next month, could have been shot and killed.”

He added: “I went out to have a look and picked up the bag and it was really heavy.

“I thought it might be full of coins and took it inside using both my hands because of its weight. I took it upstairs and put it on my bed and opened the bag up. I could not believe my eyes.”

A spokesperson for Khan said: “This is a very serious incident, which has been referred to the Met’s directorate of professional standards. The Met must now take all steps to ensure an incident like this never occurs again.”

Khan, who was MP for Tooting before defeating Zac Goldsmith to succeed Boris Johnson as mayor in 2016, has 24-hour police protection.