Environmental campaigners are holding a "tractors and trolleys" march through central London to protest against genetically-modified crops.

Protestors from Friends of the Earth will visit Downing Street, the Department for the Environment, Food and Rural Affairs, and the headquarters of the National Farmers' Union.

The campaigners aim to step up the pressure on the government ahead of Thursday's publication of the results of three-year farm trials of GM crops.

"The government has spent millions of pounds of public money on trials comparing one dodgy system with another, instead of serious research to find a sustainable way to grow maize," said campaigner Peter Riley.

"The indications are that tests on GM oil seed rape and beet will show that they have more impact on wildlife than their non-GM equivalents.

"These trials were never enough to give GM crops the green light, but they may provide enough information to give them the red one."

The protests came after former environment minister Michael Meacher described the tests as "flawed" following a European Union ban on a key herbicide used in the experiments.