Fresh doubts about the safety of an artificial sweetener have been raised by Italian scientists who have linked its use to leukaemias in rodents.

Aspartame is 200 times sweeter than sugar and is used throughout the world in low-calorie drinks and foods.

Regulators say existing studies show it is safe, but will look at the European Journal of Clinical Oncology study.

But they said it was unlikely that the sweetener was harmful to humans to the same extent as in rats.

Concerns have been raised about the aspartame in the past, but an analysis of 500 papers by UK regulators in 2002 concluded there was no threat to consumers.

The Food Standards Agency said: "The European Food Safety Authority intends to undertake an urgent assessment of this study to establish whether there are any implications for human health.

"We will study EFSA's opinion carefully and consider what, if any, action may be required."

Dr Elaine Vickers, cancer information officer at Cancer Research UK, said: "If a risk to humans does exist, it will almost certainly be very small.

"However, we welcome the news that the EFSA will undertake an urgent assessment of this work."

The study

Dr Morando Soffritti and colleagues at the Cancer Research Centre in Bologna fed eight-week-old rats varying concentrations of aspartame.

Compared with control rats given no sweetener, many of the female rats in the experiment developed lymphomas or leukaemias - the risk increasing with the dose of aspartame.

The researchers say their study raises concerns about the levels of aspartame to which humans can be exposed and, therefore, "urgent re-examination" of aspartame's safety is needed, "especially to protect children".

The existing European Food Safety Authority safety assessment for aspartame led to the setting of an Acceptable Daily Intake, or ADI.

This is an estimate of the amount of an additive that could be routinely consumed every day over a lifetime with no appreciable health risk.

'Safe' intake

Aspartame's ADI is set at 40 milligrams per kilogram of body weight. This is equivalent to 2,800 milligrams for an average British adult.

For an average three-year-old child the amount is of the order of 600 milligrams.

An adult would have to consume 14 cans of a sugar-free drink every day before reaching the ADI, assuming the sweetener was used in the drink at the maximum permitted level.

In practice, most drinks use aspartame in combination with other sweeteners so that the level is considerably lower, says the FSA.

Previous work by the former Ministry of Agriculture, Fisheries and Food and the Department of Health showed that aspartame intakes were considerably below the recommended maximum level, even among children and diabetics who consume large quantities of sugar-free drinks.