Scientists believe they have discovered why women smokers are twice as likely to develop cervical cancer than non-smokers.

They found nicotine works with a certain molecule to increase the cancer-causing effect of the sexually-transmitted infection Human Papilloma Virus.

Hpv causes around 70% of cervical cancers, which kill 1,000 UK women each year.

And though up to 80% of women will be infected with HPV at some point in their lives it usually needs no treatment and is harmless.

Dr Daniel Ndisang, from University College London which did the research, said: "These findings could help increase awareness of the necessity of avoiding smoking and even passive smoking amongst women."