The tests, carried out in North Yorkshire, saw six young people sold potentially lethal weapons without being asked their age. In Sheffield, a similar sting earlier this week resulted in two more children being sold knives.
It comes amid mounting fears over youth knife crime, and has sparked warnings over the ease of access when it comes to buying weapons.
“All it takes is for one knife to fall in the wrong hands to have a devastating impact on many lives – not just the victim but their friends, family, community and the perpetrator as well," warned Det Supt Steve Thomas, operational lead for knife crime at North Yorkshire Police.
While the test did show that more work is needed to ensure shops are following the law, he added, he praised four further retailers who were prepared to challenge the children.
"Their intervention could save a life," he said.
The tests from North Yorkshire Trading Standards saw volunteers aged 16 attempt to buy knives in 10 major retailers. Legally, stores cannot sell to under 18s, and the young people were all armed with proof of identity to show their true age, if they had been asked.
The six shops which sold the knives have not been named. They have not been subject to any prior complaints, North Yorkshire Trading Standards has said, adding that as a result no prosecutions will be made.
Instead, they have been spoken to, and issued with written advice, and the watchdog has warned it will be testing them again to check if changes have been made.
The sting came in support of a coordinated week of action across the country to raise awareness of knife crime.
Nationwide, there have been rising concerns about gang violence, youth crime, and teenage stabbings.
“It is a crime to sell age-restricted products, such as knives, to minors, so all retailers must ensure that their staff members have the training and confidence to refuse illegal sales to minors," said County Coun Andrew Lee, executive Member for Trading Standards at North Yorkshire County Council.
“While reports of underage knife sales have not been an issue in North Yorkshire, the results of these test purchases do raise concerns, particularly against the backdrop of instances of knife crime elsewhere in the country.
"Retailers have an important role to play in ensuring our young people cannot buy potentially dangerous products, so we want to see retailers complying with their legal and social obligations.”