'Irresponsible' shopkeepers have been blasted after 12 were caught selling knives, fireworks or alcohol to youngsters.
One in six stores tested during a borough-wide crackdown on underage sales, carried out ahead of Fireworks Night, fell foul of the law.
Volunteers aged between 13 and 15 visited 72 outlets, accompanied by officers, during the two-week sting operation.
They were able to buy knives in six shops, fireworks in four and alcohol in two.
Cllr Barbara Reid, Hounslow Council's lead member for environment, said: "I'm horrified there are still shops of all sizes selling restricted goods to young people, who were obviously underage, without asking them to verify their ages.
"Most worrying is that, given recent attention on the issue of knives and knife crime, it was so easy for the young people to get hold of them.
"Retailers should be asking young customers for identification so young people know that no ID means no sale."
The council regularly carries out sting operations and traders caught selling goods to underage customers face a maximum fine of £5,000 per offence.
Trading standards officers also tested how safely fireworks were stored in the shops during the sting operation.
Yobs shoved lit fireworks through the letterboxes of three homes in the space of five days last month and a shopkeeper told last week how his store was nearly destroyed in a similarly attack.
PC Clive Atkinson, from Hounslow Police, said: "The law is clear on the issues of age restricted products.
"Most local businesses have taken effective steps to prevent underage sales occurring, but there are others who need to follow suit and face up to their responsibilities.
"These products are restricted for a reason, and retailers must ensure their staff are trained on preventing underage sales and to make sure they are only selling these items to those aged 18 and over."
Residents who are aware of shops selling products like cigarettes, alcohol and knives to underage shoppers should call 020 8583 5555 or email:firstname.lastname@example.org