THE MOST widely used sweetener in the world, found in fizzy drinks and sweets, is being made using a secret genetic engineering process, which some scientists claim needs further testing for toxic side-effects.

As the G8 summit of rich country leaders decided last night to launch an inquiry into the safety of genetically modified (GM) food, an investigation by the Independent on Sunday revealed that Monsanto, the pioneering GM food giant which makes aspartame, often uses genetically engineered bacteria to produce the sweetener at its US production plants.

"We have two strains of bacteria - one is traditionally modified and one is genetically modified," said one Monsanto source. "It's got a modified enzyme. It has one amino acid different."

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The use of genetic engineering to make aspartame has stayed secret until now because there is no modified DNA in the finished product. Monsanto insists that it is completely safe.

A Monsanto spokeswoman confirmed that aspartame for the US market is made using genetic engineering. But sweetener supplied to British food producers is not. However, consumer groups say it is likely that some low-calorie products containing genetically engineered aspartame have been imported into Britain.

"Increasingly, chemical companies are using genetically engineered bacteria in their manufacturing process without telling the public," said Dr Erik Millstone, of Sussex University and the National Food Alliance.

MPs want the Government to launch an inquiry to see how much US aspartame is coming into the UK. Norman Baker, Liberal Democrat environment spokesman, will this week write to Jeff Rooker, the Food Safety minister, to ask him to ensure that US aspartame is labelled as genetically modified. "Monsanto's sweetener has turned sour," he said.

Aspartame is made by combining phenylalanine, which is naturally produced by bacteria, with another amino acid. Monsanto has genetically engineered the bacteria to make them produce more phenylalanine. Scientists fear that other unknown compounds, which may end up in food, are produced by the genetic engineering process.

"Whether such a contaminating compound will be toxic, or not is completely unknowable until empirical studies are done to test toxicity," said Dr John Fagan, a former genetic engineer who now heads Genetic ID, the world's leading GM test centre. "No such studies have been done, or at least they have not been placed in the public domain.