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The ad for the NHS smoke-free campaign showed a man who lit up a cigarette outside his house. A growth appeared on the cigarette and increased in size as he smoked

The voiceover said: "When you smoke, the chemicals you inhale cause mutations in your body and mutations are how cancer starts. Every 15 cigarettes you smoke will cause a mutation. If you could see the damage, you'd stop".

The spot attracted 18 complaints with pressure group Forest, challenging whether that claim was misleading.

The DoH said the claim was based on published, peer- reviewed scientific papers from well-respected sources with the relevant papers supplied.

The department said it had worked with two respected cancer experts who provided advice on the science around genetic mutations.

One of the experts was also one of the authors of a paper used to develop the ad.

The DoH said it had sent both experts the ad scripts and each claim was discussed with them to ensure the wording was scientifically correct. The department asked an expert in cancer genetics and genomics to respond further on its behalf.

The expert said the paper used for the basis of the claim was supported by additional research that expanded the sample size of lung cancer patients and contained detailed analysis and mutation counts.

The DoH said it had worked with broadcast regulator Clearcast who had questioned the robustness of the original research paper.

It said Clearcast subsequently discussed the claim alongside additional studies, which had larger sample sizes, with the academics and were reassured that the claim could be substantiated.

Despite the questions over the accuracy of the claims, which occurred three times, the ASA decided not to uphold the complaints as it acknowledged that the DoH had provided a range of peer-reviewed papers.

The regulator noted that the papers supported higher mutation rates among smokers versus non-smokers and was satisfied that the papers indicated that a "broadly linear dose-response relationship between mutations and cigarette smoking existed" and decided that no action to ban the ad or ask for it to be recut was necessary.

Simon Clark, director of Forest, said: "The Department of Health did everything it could to derail our complaint and were given every opportunity to do so. 

"Despite this, the ASA executive upheld our complaint three times. That speaks volumes."