The Advertising Standards Authority (ASA) has received 138 complaints since the ad, which aims to promote the benefits of learning first aid, launched on 17 September.
The ad, which was created by BBH, shows a father being diagnosed, undergoing treatment and surviving cancer and then choking to death at family party because no one around him knows basic first aid.
St John Ambulance aims to demonstrate that not knowing first aid can be as life-threatening as having cancer and encourage more people to learn the basics.
Although it was only shown after the 9pm watershed, complaints have been made that the ad is “offensive and distressing, particularly to people with cancer”.
The ASA is currently assessing whether there is grounds for a formal investigation.
Charities often face criticism over hard-hitting images and themes in ads but argue that it is necessary to raise awareness of the cause.
Barnardo’s 2008 advert showing a young girl being abused is one of the most complained about ads of all time.
Scott Jacobson, St John Ambulance’s director of marketing, communications and fundraising, says that the number of complaints is a small fraction of the 50 million people the campaign has reached.
He says: “Of course, we’re sorry to those who felt our tactics were too harsh but we hope those people realise that this campaign will save lives. The ad has been said to be shocking but then it’s shocking when someone dies who could have had a chance to live.
“Our work is vital if we’re to reduce the number of lives being needlessly lost each year. In the past week alone we’ve had tens of thousands of texts for our free first aid guide and downloads of our free first aid app. We wanted this campaign to really stun people with how needless it is for anyone to die in a situation where the most basic first aid might have saved them and that’s exactly the result we are getting.”
Earlier this year the ASA published a report carried out by Ipsos-Mori into the public’s perceptions of harm and offence in UK advertising. It found that charity ads can sometimes go too far with shock tactics.