Footage showing moped robbers targeting a mother with her three-year-old child has provoked fresh concern about the violent trend.
Footage of the incident in Petersham, south-west London, showed four male suspects riding two motorcycles and dressed all in black pull up.
The pillion passengers can be seen getting off and looking around for targets, loitering at a junction until the woman started crossing the side street with her young child.
Two of the men approached her before she scooped up her child and fled across the road into the path of an oncoming lorry, whose occupants stopped and jumped out with scaffolding poles to chase the suspects.
A CCTV clip was shared on social media by Britain’s Got Talent judge Amanda Holden, who praised the “heroes in a lorry”.
“I know this is happening everywhere,” she wrote. “Call 101 or 999 if you know these scumbags.”
Hundreds of Twitter users responded to her post, with many raising concern about “brazen” moped crime and the loss of 20,000 police officers since 2010.
A spokesperson for London's Metropolitan Police said officers were investigating the attempted robbery on 21 June.
“A woman was approached by four men on a black moped and motorbike who demanded her jewellery,” the force said in statement. "They threatened her and a young child who was with her. No injuries were reported. No weapon was seen, although it was intimated. Members of the public went to the woman's assistance and the suspects fled empty-handed.
“There have been no arrests at this time and enquiries continue.”
The incident came within weeks of comedian Michael McIntyre being targeted for his Rolex watch by a gang of hammer-wielding robbers riding mopeds outside his children’s school.
Home Secretary Sajid Javid also recently revealed he was mugged for his phone near Euston station by moped thieves while serving as community secretary.
“I was angry and upset but thought myself lucky not to have been stabbed or beaten up like many other victims who fall prey to these vicious criminals,” he said.
The use of mopeds to launch violent robberies using acid, knives and hammers has sparked alarm, seeing moped delivery drivers targeted for their vehicles, which are then used to commit more crime.
Statistics show there have been an average of 60 scooter-enabled crimes a day in London this year, although police said the rate has fallen significantly from a peak in July 2017.
Police said the decline has been aided by the deployment of new tactics such as the use of off-road bikes in pursuits and DNA marker sprays.
But identifying moped criminals has been made difficult by the wearing of helmets, masks and gloves, while some have chosen not to use head protection or remove helmets in the belief it will prevent police chasing them.
In May, the Home Office announced that officers pursuing criminals riding scooters could be given new legal protections to prevent them being prosecuted in the event of a crash.
The government said it wanted to “smash the myth that officers cannot pursue riders who are not wearing helmets”, by making it clear in law that suspects are responsible for their own decision to drive dangerously.
The technique was attempted by two moped “highwaymen” who were later jailed for stabbing a charity worker to death for his iPhone PIN during a four-hour spree of robberies.
Knife crime in England and Wales has rocketed by 22 per cent across England and Wales in a year and robberies by a third.
In London Scotland Yard’s latest statistics show a 5 per cent rise in violence in the year to April, while knife crime is up 18 per cent, muggings up 30 per cent, youth homicide up a quarter