The figures comes from test-purchasing carried out by Shropshire Council's trading standards department.
Frances Darling, the Trading Standards & Licensing Operations Manager for Shropshire Council, said the authority had, however, been encouraged there was a drop in the number of shops breaking the law on the previous year.
The number selling age-restricted items to children had fallen from 28 per cent in 2018/19, to 18 per cent in 2019/20.
The figures were revealed in a report from Councillor Gwilym Butler, the authority's Portfolio Holder for Communities, Place Planning and Regulatory Services.
The report prepared for councillors stated: "A programme of market surveillance and intelligence led age-restricted products test-purchasing exercises were undertaken in relation to the supply of alcohol, tobacco, nicotine inhaling products (e-cigarettes), knives and fireworks.
"A total of 62 retail premises were visited with 11 (18 per cent) of those selling products to under 18-year-old volunteers. This compares with 28 per cent (17 out of 60) that sold in the previous year.
"To address these offences, a range of enforcement actions have been utilised against businesses and individuals including written warnings, fixed penalties, simple cautions, prosecutions and licence reviews in accordance with the Council’s Better Regulation and Enforcement Policy."
Ms Darling said the authority would continue to root out shops that sell illegal products to children.
She said: “The council is committed to driving down the sale of age-restricted products that impact on the health, safety and well-being of children.
“Our programme of market surveillance and intelligence led test-purchasing operations that were undertaken last year focussed on alcohol, tobacco, nicotine inhaling products (e-cigarettes), knives and fireworks.
“We were obviously very encouraged by the drop in sales from the previous year from 28 per cent to 18 per cent; but equally we remain concerned that nearly one in five retailers continued to break the law by selling a range of age-restricted products to children.
“We have dealt with the businesses and individuals involved in the sales through a range of enforcement actions including written warnings, fixed penalties, simple cautions, prosecutions and licence reviews in line with the council’s 'Better Regulation and Enforcement Policy' and will continue to monitor the position through future operations and by providing advice to businesses on ways to reduce the risk of selling age-restricted products to children.”