Soft drinks containing the sweetener aspartame pack shelves at a grocery store in California, the United States. (Photo: Reuters]

The rulings are the outcome of two separate WHO expert panels, one of which flags whether there is any evidence that a substance is a potential hazard, and the other which assesses how much of a real-life risk that substance actually poses. 

Aspartame is one of the world's most popular sweeteners, used in products from Coca-Cola diet sodas to Mars' Extra chewing gum. 

In its first declaration on the additive, announced early on Friday, the International Agency for Research on Cancer (IARC) said aspartame was a "possible carcinogen". That classification means there is limited evidence a substance can cause cancer. It does not take into account how much a person would need to consume to be at risk, which is considered by a separate panel, the WHO and Food and Agriculture Organization (FAO) Joint Committee on Food Additives (JECFA). 

After undertaking its own comprehensive review, JECFA said on Friday that it did not have convincing evidence of harm caused by aspartame, and continued to recommend that people keep their consumption levels of aspartame below 40mg/kg a day.