The European Food Safety Authority is reviewing "as a matter of high priority" the results of a large new study into aspartame, the artificial sweetener consumed by millions of people worldwide and used in more than 6,000 food and drink products.

Researchers at the Ramazzini Institute for cancer research in Italy say their study shows that aspartame causes lymphomas and leukaemia in female laboratory animals "at doses very close to the acceptable daily intake for humans". The authors of the study also say that while rats fed aspartame ate less food, there was no difference in body weight between treated and untreated animals.

One of the largest manufacturers of aspartame, the Japanese multinational Ajinomoto said the allegations made by the Italian study were "not consistent with the extensive body of scientific research which already exists on aspartame".

It questioned the methods used and the record of the institute. It pointed to four previous studies into the carcinogenicity of aspartame that had found no relationship between aspartame and any form of cancer. It added that aspartame broke down in the body into the building blocks of protein that occur widely in the rest of food.

It also helped people reduce their calorie intake. This contribution to cutting obesity helped to prevent cancer, a spokesman said.

The Ramazzini Institute has sent its first results to the European Food Safety Authority. EFSA confirmed yesterday that it would be asking its expert scientific panel on food additives to review the results "as a matter of high priority, in the context of the previous extensive safety data available on aspartame".

EFSA added that until that review had taken place it did not have a basis for recommending that consumers change their diet in respect of aspartame.

Although it had been presented with an outline of the findings by the institute in June, it is still waiting for the full pathology reports from the researchers. The review will also take into account all the other studies and data available.

"This will probably take several months," an EFSA statement said.

The institute said the full data would be published in six weeks' time. Aspartame is widely used to sweeten chewing gum, soft drinks, yoghurts and desserts and other low-calorie foods, and medicines including syrups and antibiotics for children.

Aspartame has been authorised for use in foods for a long time in many countries but has "a controversial history", according to EFSA. Since its approval, the safety of aspartame and its breakdown products has been widely discussed in the press and among scientists. "Up to now aspartame has been considered safe, based on the studies available."

The new study was conducted on 1,800 rats during their full lifespan. Six different dose levels were tested against a control group. The institute said the study, which is to appear in its own publication, the European Journal of Oncology, had been peer-reviewed by seven international experts "in anticipation of controversy".

Ajinomoto said it welcomed the decision by EFSA to review the claim made by the Ramazzini Institute objectively.