Smoking appears to thin a vital part of the brain, scientists have discovered.

According to the findings, long-term smoking thins the brain's cortex. This is the outer layer of the brain in which vital cognitive functions occur, such as language, memory and perception.

Scottish and Canadian scientists looked at just over 500 men and women - making this the largest study to assess smoking and the thickness of the cortex. The average age of the participants was 73 and they included non-smokers, current smokers and ex-smokers.

All of the participants had originally been examined when they were children in the 1940s. Their recent brain health was assessed using MRI scans.

"We found that current and ex-smokers had, at age 73, many areas of thinner brain cortex than those that never smoked," said the study's lead author, Dr Sherif Karama, of McGill University in Canada.

However, he also pointed out that people who quit the habit appeared to ‘partially recover their cortical thickness for each year without smoking'.

But Dr Karama warned that this recovery process is slow and incomplete. For example, heavy ex-smokers who had quit the habit 25 years previously still had a thinner cortex than non-smokers.

The scientists acknowledged that the cortex gets thinner as people age, however their study found that smoking seems to accelerate this process. They warned that a thinner cortex is linked with cognitive decline in adults.

"Smokers should be informed that cigarettes could hasten the thinning of the brain's cortex, which could lead to cognitive deterioration. Cortical thinning seems to persist for many years after someone stops smoking," Dr Karama added.

Details of these findings are published in the journal, Molecular Psychiatry.