Retailer B&M Bargains has been fined a record £480,000 for repeatedly selling knives to children as young as 14 in east London.
Four teenagers were sent to stores in Barking and Chadwell Heath by police and trading standards officials.
In June B&M Retail admitted to three counts of selling knives to under-18s.
The guilty plea meant the company's initial fine of £720,000 was reduced by a third at Barkingside Magistrates' Court.
During sentencing District Judge Gary Lucie said B&M Bargains had failed to put in a system which would stop children buying knives.
He said: "Knife crime is at record levels across the country, particularly in London.
"One of the most concerning failures is that B&M did not implement other systems. This includes putting knives in cupboards as it does with perfume."
The court heard that two teenagers, aged 14 and 15, were sent into the Chadwell Heath B&M to purchase knives on 19 September, 2017.
Two days later the Vicarage Field store in Barking was visited by a 16-year-old who picked up a three-piece knife set.
In January a 14-year-old was sent into the same store to buy a blade.
During all of the undercover operations the teenagers were able to purchase knives completely unchallenged by staff.
B&M has 28 days to pay the £480,000 fine, plus £12,428 court costs and a £170 victim surcharge.