Nearly one in five shops in the capital - including high street chains - sold knives to children in recent tests, trading standards officers revealed.
Youngsters were able to buy weapons 148 times out of 799 test purchases, which officials said was a lower figure than in previous tests.
A third of sales were made at "well known chains" and in one instance a 14-year-old was sold a machete.
A three-month campaign will now target sales of weapons on the internet.
In 2008, 22 teenagers died in London from stab wounds.
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