An Australian professor has warned religious groups that cigarette filters may contain traces of pig's blood.
Simon Chapman said recent Dutch research has identified 185 different industrial uses for a pig - including the use of haemoglobin in cigarette filters.
The University of Sydney professor said the study offered an insight into the world of cigarette manufacturing and was likely to spark concerns for devout Muslims and Jews.
'I think that there would be some particularly devout groups who would find the idea that there were pig products in cigarettes to be very offensive,' he told the Sydney Daily Telegraph.
'The Jewish community certainly takes these matters extremely seriously and the Islamic community certainly do as well, as would many vegetarians.
'It just puts into hard relief the problem that the tobacco industry is not required to declare the ingredients of cigarettes - they say "that's our business and a trade secret".'
The research found pig haemoglobin - a blood protein - was being used to make the filters more effective at blocking toxic chemicals before they entered a smoker's lungs.
Professor Chapman said that although some tobacco companies had voluntarily published a list of the contents in their cigarettes on websites, they also noted undisclosed 'processing aids' in the finished product.
At least one brand of cigarettes sold in Greece has been confirmed to be using pig haemoglobin in its manufacturing processes, he added.
Professor Chapman said: 'If you're a smoker and you're of Islamic or Jewish faith then you'd probably want to know and there is no way of finding out.'