In a review of studies that looked at the association between smoking and HIV, British doctors said five of the six studies they analysed showed smokers had a higher chance of becoming infected.

Nine of 10 other studies in the review that tracked the progression from HIV to AIDS found no link with smoking.

"The studies identified in this systematic review indicate that while smoking might be independently associated with acquiring HIV infection, it does not appear to be related to progression to AIDS," said Dr Andrew Furber, of the South East Sheffield Primary Care Trust.

Furber and his colleagues, who reported the findings in the journal Sexually Transmitted Infections, said tobacco smoke may increase susceptibility to HIV infection by modifying a variety of immune system responses.

Research has shown that smoking is a leading cause of preventable death. It increases the risk of heart attack and stroke, respiratory problems, lung and other types of cancer.

The researchers suggest in the study that public health measures that encourage smokers to quit could also improve the effectiveness of HIV/AIDS prevention programmes.

About 40 million people worldwide are living with HIV/AIDS. Nearly 5 million were newly infected in 2005 and more than 3 million adults and children died of AIDS in the same year.