The FSA has begun an 18-month study into the artificial sweetener aspartame, which is used in a wide range of products including diet Coca-Cola and Pepsi.

Some consumers and pressure groups believe aspartame causes headaches, upset stomachs, and even suggest it could be linked to certain cancers.

The FSA stresses aspartame is safe but plans to launch a research project focusing on people who report adverse reactions. The results could be used as part of a larger EU-wide project.

"This research is not to test the safety of aspartame that is already established," said FSA chief scientist Andrew Wadge. "The study will address consumer concerns, including anecdotal reports that have linked a range of conditions to aspartame."

A 2007 review of more than 500 academic studies into the safety of aspartame found the sweetener posed no health risks. The British Medical Journal said aspartame had been "demonised unfairly" by campaign groups.

The Aspartame Information Service, a body funded by manufacturers, said it was surprised the FSA was investigating aspartame given the agency accepted the chemical was safe. It blamed "rumours circulated on the internet by scaremongers, mostly from the US".

Earlier this month Venezuela banned Coke Zero "to preserve the health of Venezuelans".