A project carried out by ADAS and Rothamsted Research shows that in the past 10 years the number of broad-leaved weed species resistant to glyphosate has risen from one - Lolium rigidum - to eight.
In addition, a further seven non-broad-leaved weed species are now resistant to the herbicide.
ADAS weed scientist and Pesticides Forum chairman James Clarke said: "Glyphosate has now been upgraded to a moderate risk of resistance. But there are some weeds that are resistant which don't tend to occur in this country.
"However, there are reported cases in Europe, such as Canadian Fleabane."
He added that herbicide resistance develops more slowly than fungicides or pesticides.
The key to combating resistance is in finding warning signs: encouraging feedback from spray operators; looking for gradual loss of control over time; and checking for plants that survive when others die.