They can be bought for just a few pounds and have been hailed as a revolutionary weapon in the fight against smoking, delivering a nicotine hit without the harmful toxins found in tobacco.

Yet the growing popularity of electronic cigarettes is causing concern in medical circles and leading to confusion among the wider public.

They come in many shapes and sizes – some are indistinguishable from cigarettes while others look like alien screwdrivers – and emit an odourless vapour.

But in many ways, these devices remain a mystery.

Little is known about their long-term effects, yet they are currently unregulated – and for some that is a worrying combination.

The British Medical Association certainly has its concerns and so too do public bodies across north London.

The Royal Free Hospital in Pond Street, Hampstead, moved quickly to ban them from its buildings, while the Whittington Hospital in Magdala Avenue, Archway, plans to do so.

A Whittington Health spokesman said: “In line with national guidelines, we do not recommend the use of e-cigarettes to stop smoking because there is insufficient evidence about their long-term safety and effectiveness in helping smokers to quit.”

Camden Council says smoking is the “biggest cause of avoidable deaths in the borough” and on average, there are 227 smoking-related deaths in Camden each year.

Yet despite the potential to reduce this tragic figure, the council remains cautious.

“The current lack of regulation means that there is little in the way of clinical evaluation meaning that their safety cannot therefore be guaranteed,” a town hall spokesman said.

About 20 per cent of Camden’s GP-registered population are smokers – some 39,816 people – but there is no data on the uptake of e-cigarettes.

However, according to the charity Action on Smoking and Health (ASH), 10 per cent of adults in London have used e-cigarettes – more than anywhere else in the country.

Cllr John Bryant, chairman of Camden Council’s health scrutiny committee, was more positive than the council’s officers.

He said: “My personal view is that they are probably healthier for you than cigarettes, and safer for other people.

“The actual content of what is inhaled is a much purer form of nicotine without all the other more dangerous elements of tobacco.”

The smoking ban does not cover these devices and for many that is a key concern.

The BMA has been lobbying for this to change while, like the Royal Free Hospital, several pubs have brought in their own embargoes, including The Washington in England’s Lane, Belsize Park.

However, The Flask pub in Hampstead is not one of them. Assistant manager Francesco Di Lalla said: “As long as they’re not bothering people, I think it’s fine. I don’t see why they should be banned.”

Martin Dockrell, director of policy and research at ASH, said there is no evidence of any danger from secondhand vapour.

He warned that extending the ban could have unintended downsides.

“The health impact of banning them in the workplace could be negative,” he said.

“We know these are smokers trying to quit or who have quit. To make them go out and use this products outside, standing with smokers – you could not think of something more likely to undermine a quit attempt.”

Mr Dockrell said a big worry is the poor quality of some devices. In certain cases, the “shoddy” chargers have been known to start fires.

“Some of them are unsafe and should not be for sale, but it’s the charger and battery that are the problem, not the nicotine,” he said. “We are also concerned that there are few controls on the content of these products. If users are to have confidence in them they need to know what’s in the vapour. They also need to be sure products will deliver the hit of nicotine they expect, and we know that this is not always the case at the moment.”

It is for these reasons that ASH has welcomed plans by the Medicines and Healthcare Products Regulatory Agency, a government agency, to regulate the devices as medicines. However, the final position on regulation remains unclear while the European Union continues to debate the matter.

Mr Dockrell said: “There is no situation where it would be better for a smoker or people around them to use real cigarettes.

“However, our position is that we want a safe and satisfying alternative to smoking for those who might want to quit or who find it hard to quit – and these products are not there yet.”