"Since then we've recognised that there's been quite a bit of tension in schools with these cards being carried around," said Inspector Paul Longden of Wiltshire Police.
In recent weeks there have been several serious incidents of "Pokémon crime".
On one occasion a Bristol toy shop was broken into and Ł300 worth of cards were stolen.
Shop owner Rachel Cardwell said: "We were basically the victims of an attack by some children, we believe. They smashed through a window making a hole which wasn't big enough for an adult to get through and they only stole Pokémon cards."
In Swindon, Wiltshire, two boys held another at knifepoint on a railway footbridge, and stole Ł200 worth of cards.
"Since then we've recognised that there's been quite a bit of tension in schools with these cards being carried around," said Inspector Paul Longden of Wiltshire Police.
"Our advice to parents is to ask children to exercise some caution. You wouldn't expect them to carry Ł200 around, so don't expect them to carry Ł200 worth of cards around," he added.
Police have urged parents not to allow children out with the cards.
But across the country, school children are literally fighting to get their hands on the cards of the 151 Pokémon characters, based on the Japanese cartoon, film and computer game.
Many schools have banned the cards. One school in Cheddar, Somerset, has started an afterschool swap club to avoid playground tussles.
But the crime wave persists.
"At school people go around nicking other people's cards, I got five of mine nicked and they were all my good ones," said one schoolboy.
And with the Pokémon industry worth a cool Ł4bn, counterfeiters are also after a piece of the action.
Fakes of cards not yet released here are coming into Britain from Hong Kong and America and are not always easy to spot.