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E-cigarettes can cause cancer even when they are nicotine-free, scientists have claimed.

The research suggests that there might be little difference health-wise between using the electronic pens and smoking regular cigarettes .

Lab experiments on the battery operated devices found they lead to the same kind of cell damage that triggers the disease.

The finding adds to growing evidence electronic cigarettes may cause serious health problems because of the toxins they contain.

In the study scientists tested two popular brands and showed they harmed cells in ways that could spark the development of a tumour even with no nicotine, the addictive ingredient in tobacco.

Writing in the journal Oral Oncology, they said: "Our study strongly suggests electronic cigarettes are not as safe as their marketing makes them appear to the public."

Although some countries have banned e-cigs their use and sale in the UK, like the US, are legal.

(Image: Getty)

But the US Food and Drug Administration does not regulate e-cigarettes and has warned of possible health risks, although evidence is limited as they have just been readily available for less than a decade.

Professor Jessica Wang-Rodriquez, of the University of California , San Diego, said: "There haven't been many good lab studies on the effects of these products on actual human cells."

The head and neck cancer specialist and colleagues at the Veterans Affairs San Diego Healthcare System created an extract from the vapour of the products and used it to treat human cells in petri dishes.

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These were more likely to suffer DNA damage and die, compared with untreated cells.

In particular the exposed cells showed tell tale 'strand breaks' in the familiar double helix of molecules that intertwine.

When one or both break apart and the cellular repair process does not work properly, the stage is set for cancer.

The affected cells were also more likely to launch into programmed cell death or cell injury, known as apoptosis and necrosis respectively.

(Image: Corbis)

In the main part of the study the researchers used normal epithelial cells which line organs, glands, and cavities throughout the body, including the mouth and lungs, and found while nicotine caused worse damage, vapour that was free of the chemical was enough to alter them.

Prof Wang-Rodriguez said: "There have been many studies showing nicotine can damage cells.

But we found other variables can do damage as well.

"It is not that the nicotine is completely innocent in the mix, but it looks like the amount of nicotine the cells are exposed to by e-cigarettes is not sufficient by itself to cause these changes.

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"There must be other components in the e-cigarettes that are doing this damage. So we may be identifying other carcinogenic components that are previously undescribed."

Her researchers are now trying to sort out those other substances and their specific effects.

Scientists already know of some troubling chemicals in the products. One is formaldehyde, a known carcinogen.

Using the products at a low voltage setting may minimise the production of formaldehyde, research suggests.

(Image: REX/Tony Kyriacou)

Another possible culprit is diacetyl, a flavouring that has been linked to lung disease.

A Harvard study found it in more than three quarters of flavoured e-cigarettes and refill liquids, or 'e-juice'.

There are nearly 500 brands of e-cigarettes on the market, in more than 7,000 flavours, so scientists have their work cut out for them identifying all the potential problems.

Prof Wang-Rodriguez said: "For now, we were able to at least identify that e-cigarettes on the whole have something to do with increased cell death. We hope to identify the individual components that are contributing to the effect."

While accepting lab cells are not completely comparable to those within a living person and the dose did not mimic what an e-cigarette user would get, she said the overarching question is whether e-cigs are really any safer than the conventional cigarettes they are designed to replace.

Added Prof Wang-Rodriquez: "Based on the evidence to date, I believe they are no better than smoking regular cigarettes."

Instead of burning tobacco, e-cigs generally contain cartridges filled with nicotine and other chemicals.

When it is used, the liquid chemicals in the cartridge are turned into a vapour or steam that is inhaled by the smoker.

It is estimated there are currently 2.6 million adults in Great Britain using electronic cigarettes, while regular users in the US number roughly 12.5 million.