The researchers - drawn from several universities including Cambridge, Oxford and Kings's College in London - said the findings should be used to reassure smokers attempting to quit that concerns about increased anxiety levels were unfounded.

BBC: Quitting smoking 'reduces anxiety'

The Epitwin Project is the brainchild of Tim Spector, who heads the Twin Research Unit at St Thomas's and is Professor of Genetic Epidemiology at Kings College London.

BBC: 'Not-so-identical' twins may hold the key to disease