In the first prosecution of its kind in Britain, the Health and Safety Executive charged the company with failing to prevent pollen from genetically modified crops from being released into the environment at a trial site in Lincolnshire.
Monsanto admitted the offence and was also ordered by magistrates at Caistor, Lincolnshire, to pay pounds 6,159 costs.
Another company charged with a similar offence at the same site, the seed producer Perryfields Holdings, was fined pounds 14,000 and ordered to pay costs of pounds 5,000.
The case was another setback for the Government in its attempt to convince the public that genetically modified food plants present no risk, in cultivation or consumption.
The Food Safety minister, Jeff Rooker, insisted after the hearing that there was a "robust" regulatory system in place and that consumer protection was the Government's top priority, but Friends of the Earth denounced Monsanto's fine as "pathetic," and William Hague, the Leader of the Opposition, said that a Conservative peer would introduce a private member's Bill in the Lords to impose a three-year GM crops moratorium.
English Nature, the Government's wildlife adviser, has called for such a delay before such crops are grown commercially because of the dangers to wildlife of the new weedkillers developed to go with them.
Monsanto said it would resist any such move, if necessary by appealing to the European Commission.
The court heard that both Monsanto and Perryfields were using the plot at Rothwell to test-grow varieties of oilseed rape that had been genetically modified to be tolerant of particular weedkillers.
A condition of their licences was to keep a six-metre-wide pollen barrier of non-GM crops around the modified crops, to prevent modified pollen mixing with normal plants in the area.
But an HSE inspection found that the pollen barrier had been cut back in some places to just two metres to put in a roadway, and improve the look of the site.
An HSE inspector, Andrew Tommey, also found a gap had been created between the GM crops and the pollen-barrier crops, creating what he described as a "wind tunnel", allowing pollen to escape.
Simon Parrington, for the prosecution, said no one from either company bothered to visit the site to see if the terms of their licences were being followed.
"Neither company had taken sufficient steps to make sure that the barriers were in place," he said.
The companies said the pollen barrier had been cut back by an employee of the firm hired to manage the site.
Rhodri Price Lewis, for the defence of both companies, told the court: "An employee who was not aware of the consent regulations mowed and removed plants in order to make it easier to get at the trial plants. This was not an act which was under the control of these companies."
A concern of ecologists is that pollen from herbicide- tolerant crops might carry weedkiller resistance into nearby wild plants, so producing "superweeds."
But Dan Verakis of Monsanto said after the hearing that the chance of GM pollen affecting other crops in the area was minimal.
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