Northern Ireland police are distributing marijuana-smelling scratch-and-sniff cards to help people recognize the smell of weed being grown illegally.

When scratched, the cards produce the scent of cannabis in its "growing state," says the Guardian newspaper, which adds the odour is different from the smell of marijuana smoke.

The cards' cannabis odour is caused by a chemical, not the plant itself.

The money generated from illegal cannabis farms runs into millions of pounds, say North Ireland police, who are handing out marijuana scratch-and-sniff cards to the public to help alert them to grow-ops. (CBC)

"More than two cannabis factories are being uncovered every week in Northern Island," the Police Service of Northern Ireland says in a video release.

"The money generated by them runs into millions of pounds, and that's money going into the pocket so organized criminals used to fund even more criminal activity."

The initiative comes in the wake of a 44 per cent increase in Northern Ireland's cannabis factories from 2013 to 2014, the magazine The Week reports at its website.

The PSNI video notes that the unregulated electricity and water used in the marijuana factories are "a potentially lethal mix."