More than 50% of shoppers buying age-restricted purchases online, including knives, were not asked to prove their age.
More than 4,100 teens took part in secret audits of bricks and mortar stores last year, with homeware stores faring the worst under scrutiny.
A quarter (26%) of all secret shoppers were not asked for ID at homeware stores, while 15% managed to buy a knife without their age being verified at supermarkets.
Offences involving knives or sharp instruments increased by 6% in 2018 compared to the previous 12 months, with a decade-high of more than 730 homicides being recorded in England in Wales last year.
The North West and Midlands had the lowest rate of ID checks, with 21% of shoppers not being ID'd in 2018.
They were followed by East Anglia and the North East, in joint second place with 18% not being checked.
London was the most stringent, but still saw more than one in ten shoppers not being checked (13%).
Checks on purchases online are starkly worse compared to the high street.
Half (51%) of the 4,395 young mystery shoppers received age-restricted goods, including knives, they ordered online without being asked for proof of age.
Ed Heaver, director of Serve Legal, the leading provider of compliance auditing services who published the results, told Sky News the 'gold standard' retailers should aim for is ID checks being carrier out on 90% or more of customers, allowing for some human error.
The shoppers used by Serve Legal are "young looking 18 and 19-year-olds", he said.
Although the numbers of shoppers not being checked remains high, the most recent figures are an improvement on previous years.
In 2018, 18% of mystery knife buyers were not asked to show ID. This is compared to 25% in 2017 and 43% five years ago.
Homeware stores improved their pass rates from 59% in 2017 to 74% in 2018.
Mr Heaver said: "Bricks and mortar retailers really upped their game over the last 18 months… They can do more and improve further but they are getting closer to [age checks for] alcohol levels… in the upper 80s%".
Mr Heaver said: "[Stores] should get recognition for focusing on this area… but the danger is if they get complacent and shift focus."
He added that carrying ID is now "a part of young people's lives" as "it is not as though [checks] are a recent thing."
The Home Office told Sky News: "We are making it harder than ever for young people to purchase knives, including online.
"As part of the serious violence strategy we are supporting trading standards through our prosecution fund to help them to prosecute retailers who sell knives to under 18s and the fund has already supported enforcement activity in more than 1,100 cases.
"The Offensive Weapons Act, which received royal assent on 16 May, will ban knives being sent to residential addresses after they are bought online, unless the seller has an arrangement to verify age at the point of delivery."
In England and Wales, it is illegal to sell a knife, blade or axe to a person under the age of 18. In Scotland sales are allowed to children aged 16 and over.
Many retailers have a Challenge 21 or Challenge 25 policy, which requires staff to check the ID of all customers who appear to be 21 or 25 years of age or younger.
The issue of knife crime appears to be taken seriously by the general public.
A survey of 2,000 people carried out by Censuswife and commissioned by Serve Legal found one in three would report a store to police or Trading Standards if it was selling age-restricted products like knives to young people without age checks.
One in 10 would boycott the store altogether.
The regions ranked by the percentage of secret shoppers whose age was not verified when buying a knife in a store last year. Samples for NI, Eire and other are statistically unreliable because they were too small and so have not been included.
1= North west - 21%
1= Midlands - 21%
2= East Anglia - 18%
2= North east - 18%
3= Wales - 17%
3= Scotland - 17%
4 South west - 16%
5= South east - 14%
5= South central 14%
6 London - 13%
The regions ranked by the percentage of secret shoppers whose age was not verified when buying a knife in a store in 2017. Samples for NI, Eire and other are statistically unreliable because they were too small and so have not been included.
1 Scotland - 41%
2 Midlands - 30%
3 Wales - 29%
4= North west - 26%
4= East Anglia - 26%
4= North east - 26%
4= South west - 26%
5 South central - 23%
6 South east - 22%
7 London - 18%