Electronic cigarettes are more dangerous than experts previously thought, it has emerged.

The devices contain one million times more harmful substances than polluted air - including cancer-causing substances and flame retardants.

Furthermore, scientists have discovered for the first time some contain toxins linked to fertility problems, fetal development issues and thyroid disruption.

The study, by researchers at Baptist University in Hong Kong has prompted health officials to call for an immediate and total ban on all e-cigarettes 

The Hong Kong Council on Smoking and Health said the potential health risk of the devices must not be underrated.

Experts at the council said because most e-cigarettes that are for sale do not provide information on their components they commissioned a laboratory test by university researchers.

Chairman of the council, Antonio Kwong said: 'The test results showed the quality of e-cigarettes varied and carcinogens and other harmful substances were found.

'To nip it in the bud and protect public health, we urge the Government to totally ban e-cigarettes promptly.'

Researchers at Baptist University analyzed 13 different types of the device, which have seen a surge in popularity in recent years.

Those tests confirmed the presence of harmful chemicals, including formaldehyde and polycyclic aromatic hydrocarbons (PAHs) - both of which are known carcinogens and are hazardous to health.

Furthermore the scientists noted a significant variation in the components of e-cigarettes, even those in the same batch of the same brand.

Dr Chung Shan-shan, assistant professor of the department of biology, said not only were carcinogens found, but flame retardants - poly-brominated diphenyl ethers (PBDEs) - used in electronic products.

He said: 'PBDEs, effective flame retardants extensively used in electronic products, were first discovered in e-cigarettes.

'We believe that PBDEs were added to reduce the rate of combustion of the combustible component in e-cigarettes, eg. plastics.

'Inhalation of PBDEs has been associated with thyroid hormone disruption, reduction of fertility, affect fetal development and can cause cancers.' 

And he told the South China Morning Post: 'The [Level of PAHs] in e-cigarettes is at least one million times more than roadside air in Hong Kong.'

A wide variety of e-cigarettes are now available in Hong Kong via various sale channels and are mainly targeting the youngsters. 

However, most of them do not provide details on their components nor carry any health warnings. 

Some e-cigarettes are even marketed as 'healthy', 'non-carcinogenic', 'non-toxic', 'safe' or 'no harm' which mislead consumers. 

Users inhale unknown substances will bring potential health risks.

The safety and effectiveness of e-cigarettes as a smoking cessation aid are unknown. 

Insufficient evidence has been identified so far to support the claim that e-cigarettes help smokers kick the habit, says the World Health Organization (WHO).

At least 16 countries have imposed a total ban on e-cigarettes, including Singapore, Thailand and Brazil.

The Council on Smoking and Health is calling on Hong Kong's government to enact a total ban on the sales, advertising, promotion and sponsorship, distribution, import and manufacturing of e-cigarettes.