Britain’s burgeoning vaping industry is warning of a rise in homemade versions of the liquids used in the devices as new laws governing their strength take effect this weekend.

Vape shops warn that the health of consumers will be put at risk because people will end up buying stronger products from the black market or the internet that do not meet safety standards. There are also concerns that thousands of vapers who use high levels of nicotine will end up reverting to cigarettes now they can no longer buy sufficiently high doses under the new rules.

The Tobacco Products Directive, which came into force on Saturday, is most widely known for prohibiting the selling of cigarettes in branded packs. But it also makes a number of demands on vaping companies and retailers.

The new rules include restrictions on the size of the e-cigarette tanks and refill containers that vapers can use. From Saturday UK vapers can buy devices that have tanks with a maximum size of 2ml – far smaller than most on the market – which means they will have to refill more frequently. They will also be able to buy only small bottles of e-liquids, meaning they will have to carry several around with them during the day.

“If you are continuing to have to refill, it’s annoying that you now have to carry more stuff with you,” said Richard Hyslop, chief executive of the Independent British Vape Trade Association, which represents about half of the UK’s 1,700 vaping shops. “From a public health point of view, vaping succeeds or fails on the number of smokers who switch to it,” he said. “If you’re looking at vaping compared to smoking, with smoking you just take a cigarette out of the packet and light it. With vaping you’ve got to have all this kit and keep refilling because the tank’s so small and the bottle size is so much smaller.

“There’s already an informal economy in cloned devices and poor-quality stuff. Now people wanting to buy devices with bigger tanks or liquid in a bottle that’s bigger than 10ml won’t be able to buy it legitimately in the UK but will be able to go to grey or informal areas and buy it.”

Around 6% of Britain’s 2.9 million vapers are using e-liquids that contain more than 20mg of nicotine per ml and which are now also banned.

Faced with restrictions on nicotine strength, Hyslop predicted the UK’s vapers would consider a number of options including buying e-liquids in bulk off the internet and looking to dilute it. “Those customers who need a high-strength product will either look to make it themselves, go back to smoking or seek out the informal [black market] economy which is not tested.”

Public Health England recently produced a report suggesting vaping was 95% less harmful than smoking. But vaping’s success in weaning heavy smokers off cigarettes was now in jeopardy, Hyslop said. “Many vapers start on a high level and go down the nicotine strength and end up on zero but heavy smokers now will struggle to make the switch.”

Vape shops claim a rush to regulation could end up stifling the industry, which is less than 10 years old. Hyslop drew comparisons with Italy, which introduced punitive taxes and saw its domestic market collapse. But France has placed no restrictions on tank size.

Shayne Adams, managing director of the Flavour Vapour chain of shops, predicted that some retailers would go out of business. “These shops are private-sector stop-smoking centres and many will be forced to close due to this financial hammer blow,” he said.

On Friday the New Nicotine Alliance, led by Professor Gerry Stimson, a public health expert, protested against the new measures outside the Department of Health. “Vaping has helped 1.5 million give up smoking,” Stimson said. “This extraordinary success is put at risk by rules that make vaping less attractive to Britain’s 9 million current smokers.”