The Head of Westminster’s Neighbourhood Crime Reduction Service Mick Smith said:
“The swindle works by fraudsters dropping a gold coloured ring next to an unsuspecting passer-by.
“They then ‘spot’ the ring and make a big show of picking it up, asking their mark if it is theirs.
“When the person says no, they offer to sell it to them for anything up to £20 usually claiming they need money for food.
“Often they will point to a fake hallmark – which we suspect they have put in themselves.
“Obviously when the person goes to get it valued they discover that it is worthless.
“It is impossible to know how many times they have already carried this out.
“But the fact that one nearby jewellery shop alone says they have seen around 50 people in the space of a week gives you some idea as to the scale of it.”
More top news
Police investigating the Grenfell Tower fire have "reasonable grounds" to suspect corporate manslaughter may have been committed.
Charlie Gard will "inevitably" die shortly after being moved to a hospice and having life-support treatment withdrawn.
The service has 1,366 people rammed into a dozen carriages designed for just 640.