"It causes more eating damage and cannot be controlled with existing measures," he added.
The slug was first reported in the UK in early spring of 2012 by Dr Ian Bedford, head of entomology at the John Innes Centre (JIC).
Thought to have entered the UK on imported plants, the large orange-to-brown slugs are resistant to many slug pellets, are voracious and omnivorous eaters, tolerate drier conditions and reproduce in greater numbers than native slugs.
Now the JIC has called for the public to report sightings of the slug on the website www.slugwatch.co.uk.
"The reports may also give us an idea of whether it is breeding with our native species to form a hybrid, combining the worst of the Spanish slug with tolerance to cold from our own species," Bedford explained.