A warning has been fired to traders by Tower Hamlets Council to stop selling knives over the counter to teenagers after a shop was caught out during a sting operation on the Isle of Dogs.
Deputy mayor Asma Begum urged them to check youngsters' identities and to refuse purchases if in doubt.
"Shopkeepers need to be aware that selling knives to children can have serious consequences," Cllr Begum said.
"We also urge people to report any knives found or seen on the streets."
A bread knife was sold at Docklands Halal Grocers in Castalia Square, Cubitt Town, during a test purchase by two girl cadets aged 14 and 15 and witnessed by a council trading standards officer.
The cadets were posing as customers, but no attempt was made to ask their age or identity, Thames magistrates were told.
The company, REH Investments, was fined £158 with £1,561 and on August 29. The man who sold the knife, Razzak Miah, 39, was fined £130 with £115 costs at an earlier hearing after admitting selling it to someone under 18.
The shop has since stopped selling knives.