Glyphosate is believed to be the most effective herbicide for clearing vegetation before planting crops, and it is also used in domestic gardens, car parks, pavements, vineyards and orchards.

But scientists have now confirmed that the weedkiller was not effective on Italian ryegrass found in multiple fields on a farm in Kent.

They are now briefing the farming community about the development while biosecurity measures have been ramped up in the local area.

Glyphosate-resistant weeds have been detected across the world numerous times in the past, but this is the first time a case has been identified in the UK.

While unlikely to affect food prices, herbicide resistance could have a significant financial impact on farmers.

John Cussans, a weed science consultant who identified and confirmed the Kent case, said the resistant species is “unlikely to spread” because herbicide resistance was “probably caused by natural selection”.

But scientists still expect to detect more cases of randomly mutated Italian ryegrass on British farms in the short term as they increase monitoring.

Mr Cussans said the impact could be “massively consequential” on some farmers and their businesses, who may be forced to shift towards more costly weed control methods and face losing access to sustainability-focused subsidies.

Dr Helen Metcalfe, agricultural ecologist at Rothamsted Research, said the Kent case highlighted the “importance of moving away from overreliance on the herbicide as a method for controlling weeds”.

She said farmers can still use the weedkiller as “one of the tools in the toolkit, rather than as your main weed control method.

“It’s quite key that we act now to maintain the safe stewardship of this chemical, and this is our warning sign to start doing that.”