There is no evidence of an increased cancer risk from plans for bakers in New Zealand and Australia to fortify their bread with folic acid, a leading scientist says.

A still unpublished study "shows that there is no increase in cancer risk with high-dose folic acid," said Otago University Professor Murray Skeaff, a human nutrition specialist.

Prime Minister John Key has said the fortification of breads with folic acid is likely to go ahead in September but may be stopped soon after.

It was ordered last year to prevent neural tube defects in babies such as spina bifida, protecting women unaware they might be pregnant.

Bakers have criticised the move, complaining that it adds unreasonable costs, and Food Safety Minister Kate Wilkinson has called for a review - but not until October, a month after bakers have started adding it to bread.

Critics of the "mass medication" have claimed that though added folic acid will reduce birth defects - and may prevent premature birth and heart defects - extra-high levels may also pose a cancer risk to old people.

Animal studies show too much folic acid supplements in pills can spur some cancers.

And one study which tracked 640 men found that 10 years later, folic acid users were more likely to have developed prostate cancer.

Professor Skeaff said in terms of cancer risk "there is no reason the proposed fortification should not go ahead".

At a conference in Prague two weeks ago, a pooled analysis of all the randomised control trials of folic acid was revealed by the clinical trials service unit at Oxford in Britain.

"We can be confident on the basis of these trials that there is no evidence of cancer risk," Professor Skeaff said.

Lyall Thurston, speaking for a coalition of parents of children with spina bifida, said the planned supplements would reduce the number of babies born with neural tube defects.

He said the campaign against them was being funded for commercial reasons.