A third of smokers admit sabotaging another person's attempt to quit, according to a study by a pharmaceutical company.
Jealousy, guilt about their own habit and wanting a smoking "buddy" are among the reasons to wreck other people's quitting attempts.
Data collated from 6,300 current and former smokers by Pfizer shows 31 per cent of smokers admit being saboteurs.
The study also found that 72 per cent of smokers who have tried to quit think someone has tried to ruin their attempts.
On average, smokers said they tried to kick the habit at least three times. One in five said they had tried five times or more.
London-based GP Sarah Jarvis said: "Beating a smoking addiction is hard enough without the negative influence of others around you casting doubt.
"I want those who are motivated to give up smoking to be aware that they don't have to go it alone and that there is support available.
"Even a brief conversation with their healthcare professional ... can increase their chances of success by up to four times, compared with going cold turkey."