Australia could lose agricultural export dollars, and farmers could find themselves using more herbicides to control weeds and being sued by other farmers for crop contamination if they chose to grow genetically engineered crops, said Charles Benbrook, who worked as an agricultural adviser to the Carter, Reagan and Clinton administrations.

Dr Benbrook is touring Australia to warn government ministers and farmers about what he believes are the problems with the first decade of genetically modified crops in the US. His tour is sponsored by GeneEthics, a group campaigning against the release of genetically modified contaminated material.

"Across the south-eastern US, where soybean and cotton farmers have relied almost exclusively on [genetic engineering] technology for several years, the system is on the brink of collapse, the volume of herbicides used is setting new records and farmers' profit margins are shrinking," he said.

Most genetically modified crops are designed so farmers can spray their fields with herbicide, killing weeds but not the crop.

Dr Benbrook said the widespread use of genetically modified crops initially led to a drop in the herbicides US farmers used. But farmers with such crops were now using more weed chemicals than were conventional farmers.

"The increase is getting bigger as weeds become more resistant. It has definitely not been an economic boon for farmers," he said, adding that resistance in some markets, such as Europe, to genetically modified products had damaged US agricultural exports.

"What I am urging agricultural leaders and politicians to do is learn from the lessons in the US."

But Dr Ian Edwards, a spokesman for the biotechnology industry body AusBiotech, accused Dr Benbrook of "cherry picking" his statistics to suit his argument. "To blame GM crops for weed resistance has no basis in science."

Weed resistance to herbicides had developed because farmers misused or over-used the chemicals, he said.

He said farmers could always change the type of herbicide they used to avoid problems, although he conceded there was a chance weeds would eventually become resistant to the new herbicide.