A common pesticide blamed for causing cancer is being targeted by “brandalism” protests in garden centres across Scotland.

Activists are fixing new labels to ‘Roundup’ weedkiller, made by the US agrochemical giant Monsanto, warning that it “probably” causes cancer and “degrades farmers’ power”.

Roundup contains one of the world’s most widely used pesticides, glyphosate. The European Union (EU) is currently considering whether or not to re-licence its use, with crunch decisions by experts expected on May 18-19.

In March 2015 the World Health Organisation’s International Agency for Research on Cancer classified glyphosate as “probably carcinogenic to humans”. A study last month concluded that people exposed to glyphosate had an increased risk of skin cancer.

Glyphosate has also been controversial because it is designed to be used on crops that have been genetically modified to resist it. In April the European Parliament urged the European Commission to curtail its use because of health concerns, though leaks since suggest that the commission may largely ignore the parliament’s recommendations.

The chemical’s toxicity has been disputed by Monsanto and other major pesticide companies. They stress its importance for farmers and gardeners, and point to studies suggesting that it is safe.

The garden centre protests are being co-ordinated by the campaign group, Global Justice Now. “There’s no question that glyphosate should be banned,” said the group’s Scottish spokeswoman, Liz Murray.

“The reason that the EU is considering relicensing a chemical that has such potentially fatal consequences for human health is because of the enormous power that big corporations have in influencing such decisions. People are relabeling Roundup bottles in garden centres and DIY stores across the country because they want to expose its toxic contents and the massive corporate power of Monsanto.”

One activist in Glasgow, university researcher Steve Rolfe, said he was really worried that the EU would not take any notice of public health concerns. “It feels like just another example of corporate power over-riding democracy and common sense,” he told the Sunday Herald.

“In Glasgow we'll be rebranding Roundup products in garden centres and DIY shops over the next couple of weeks, so that the public can see what they're really buying. The brandalism message will highlight the cancer risk and our concerns about excessive corporate power in food production.”

Global Justice Now has produced bright yellow labels matching those on Roundup, but with very different text. “The World Health Organisation classifies this product as probably causing cancer,” they say. “Degrades farmers’ power so corporations grow.”

Professor Andrew Watterson, an occupational and environmental health expert from the University of Stirling, warned that glyphosate posed serious potential health risks. He criticised the agrochemical industry for lobbying to allow its continued use.

He was concerned that the industry would be backed by the UK government. “This will be of little comfort to agricultural and other workers and consumers who may continue to be exposed to a pesticide present in air, water and food,” he said.

“It may well, however, comfort pesticide manufacturers and large farmers who will profit from the use of a pesticide that in some formulations has been specially designed for GM crops.”

Monsanto, however, insisted that glyphosate did not cause cancer, and can be safely used by farmers and gardeners. “This view is backed up by all independent pesticide regulators with responsibility for scientific assessment of glyphosate,” said the company’s UK spokesman, Mark Buckingham.

He questioned the credibility of the WHO agency's findings, pointing out that it had also suggested that alcohol and processed meats such bacon and sausage were more hazardous. “Glyphosate is an essential tool for sustainable agriculture,” he argued.

“By choosing to campaign against one of the safest weed control tools, campaigners are effectively promoting riskier alternatives. Some campaigners suggest a switch to weed control with vinegar/acetic acid based products, which were found to be dangerous for bees and mammals.”

The Crop Protection Association, which represents pesticide companies, attacked campaigners as “misguided”. It was “unfortunate” that the arguments had become politicised, according to the association’s chief executive, Nick von Westenholz.

“Over 40 years of robust scientific evidence has confirmed that glyphosate poses no unacceptable risk to human health,” he added. “In fact, no regulatory agency in the world classifies glyphosate as a carcinogen.”