Almost 40% of sellers targeted by councils in England have been caught illegally allowing children to buy e-cigarette products, a report has found.
Ninety of the 227 premises tested sold vaping goods to under-age teenagers in 2018-19, data from 34 councils showed.
Trading Standards - which compiled the research - has called for greater resources to enforce the law.
Public Health England said vaping was 95% healthier than smoking.
It is estimated 3.6 million people in the UK now use e-cigarettes.
'Liquid in my lungs'
Georgia 17, told the BBC's Victoria Derbyshire programme she had been to shops where she had been given free e-cigarette tutorials and liquids.
She has been vaping for over a year and "rarely" been asked for ID.
Asked for proof of age the first time she had tried to buy vaping products, she had been allowed to complete the purchase after telling the seller she had forgotten it, she said.
The teenager now believes she has experienced negative health effects as a result of vaping.
"I went to hospital, they took my blood and they said there's too much liquid in my lungs," she said.
"They gave me some tablets for a course of two weeks. I'm still getting the pain today."
The Chartered Trading Standards Institute's 2018-19 Tobacco Control Survey shows the proportion of targeted sellers allowing children to buy vaping products has risen 12 percentage points in a year, from 28% to 40%.
Forty-seven of the 90 illegal sales were recorded at specialist e-cigarette suppliers.
Discount shops had the second most illegal sales, 11, while eight were reported at market and car-boot sales.
The figures also suggest sellers are more than twice as likely to allow under-age purchases of e-cigarette products compared with traditional tobacco products, for which 18% of those tested were found to have acted illegally.
E-cigarette products can also be purchased online and the Victoria Derbyshire programme has spoken to several parents whose children have bought products while under-age.
One mother, whose 13-year-old son bought them on eBay, said it had been "scarily easy" for him to do so.
She said she felt "frustration and anger" towards the retailer because there had not been any age-verification checks.
"We think probably there were terms and conditions he was signing up to when he opened his eBay account, but probably just a tick box, nothing more than that," she added.
An eBay representative said its sellers "are required to have an effective age-verification process to prevent the sale of e-cigarettes to minors".
"Sellers who do not comply will be permanently suspended and we have banned those referenced by the BBC," the representative said.
"We have also launched an additional review of seller processes and practices to ensure businesses selling these products are carrying out the required age checks on buyers."
Leon Livermore, chief executive of the Chartered Trading Standards Institute, said it "can only do so much" with its current level of resources.
"I'd say to the government, 'If you want your policies delivered effectively, you need to provide appropriate funding and resources through to the front line,'" he said.
He said he was also in favour of banning flavoured products, which some say lead children who do not smoke into vaping.
But this is something Public Health England (PHE) rejects.
Its lead on tobacco control, Martin Dockrell, told the BBC there is "no evidence that flavours are leading kids who don't smoke into vaping, but there is evidence that they are part of what helps smokers to switch".
PHE added: "There is widespread academic and clinical consensus that while not without risk, vaping is far less harmful than smoking."
'Attracted to flavours'
Sydney, 17, who began using e-cigarettes aged 16 despite not having previously smoked, told the Victoria Derbyshire programme it was the flavours she was "mainly attracted to" and she would "probably" stop were they taken away.
She said she had begun to spot the signs of addiction.
"I can go days without it but then sometimes I will have that feeling of, 'Oh my God, I feel like I do need the nicotine and the vape,'" she said.
Both Georgia and Sydney also said they had been originally attracted to vaping because of images on social media.
They explained it was becoming a "trend" led by celebrities.
A recent study by King's College London, which looked at online surveys from more than 12,000 16- to 19-year-olds worldwide, found 38% of English respondents said vaping adverts made e-cigarettes seem appealing.
This is despite regulations saying adverts can be targeted towards adult consumers only.
The UK Vaping Industry Association (UKVIA) told the BBC: "Products should never be marketed or sold to under-18s.
"We expect the highest standards from our members to show leadership to the rest of the industry.
"We also expect our members to make sure that products are not designed to appeal specifically to anyone under 18 years old," it added.