Planning to quit smoking? Be advised that there may be subsequent gain in weight in the process, according to results by a comprehensive study conducted by British and French experts.

But the undesirable side-effects were but temporary, medical doctors said, and should not discourage smokers from finally kicking the habit, which extensive researches have been blaming for numerous deaths due to heart and blood pressure complications and lung cancer.

Published by the British Medical Journal on Tuesday, the new study said that from analyses of 62 clinical trials, majority of those who lighted their last cigarette stick from a year ago put on more weight by as much as 10 kilograms.

The study covered samples from North America, Europe, Australia and East Asia though the latter only represented quite small fraction of the entire study, the analysis of which was conducted after a given smoker had been free of the habit by at least 12 months.

The average gain observed from the samples was five kilograms, which were seen on about 34 percent of the trials while some 14 percent added heft of 10 kilograms or more.

Another 38 percent registered weight gains of between five to 10 kilograms while 21 percent, the study said, shed some kilos, the quantity of which was unspecified.

The findings were definitely noteworthy because they could serve as major deterrent for people wishing to stop the deadly habit, which in Australia alone causes the death of thousands and compels health authorities to spend billions for treatment of serious medical conditions linked to smoking, medical expert said.

Yet according to Simon Chapman of the Sydney School of Public Health, the data offered by the new study was not applicable to everyone, noting that the results were obtained from specific cases of individuals that sought medical assistance following their decision to get away from cigarettes.

"So these results may not be generalisable to all smokers who quit because two-thirds to three-quarters of ex-smokers stop smoking without professional help or interventions," Professor Chapman told the Australian Associated Press (AAP) on Wednesday.

"Those who decide they need help to stop smoking tend to lack self-efficacy," he added.

It should be comfort for quitters, Professor Chapman stressed, that other major studies on the matter led to solid suggestions that weight gains as the immediate aftermath of giving up on the habit did not necessarily raised the chances of dying.

Despite the dreaded outcome, at least for most, delivered by the international study, the Heart Foundation said that smokers should still consider the fact that they face greater danger of dying from stroke and heart attack as against to weight gains that can be lost anyway with proper exercise and diet.

"Giving up smoking is the single biggest thing you can do to improve your heart health and, while being overweight is also a risk factor for heart disease, the health benefits of quitting far outweigh a few extra kilograms," Dr Robert Grenfell of the Heart Foundation told AAP.