PET, the strong plastic commonly used in bottles, takes hundreds of years to break down in the environment.
The modified enzyme, known as PETase, can start breaking down the same material in just a few days.
This could revolutionise the recycling process, allowing plastics to be re-used more effectively.
UK consumers use around 13 billion plastic drinks bottles a year but more than three billion are not recycled.
Found in a dump
Originally discovered in Japan, the enzyme is produced by a bacterium which "eats" PET.
Ideonella sakaiensis uses the plastic as its major energy source.
Researchers reported in 2016 that they had found the strain living in sediments at a bottle recycling site in the port city of Sakai.