''Scratch and sniff'' cannabis cards handed out to help members of the public detect drug farms resulted in a 28% increase in reports of cultivation.

Over 200,000 cards were distributed in the Crimestoppers campaign to hot spots including London, Greater Manchester, Merseyside, South Yorkshire and Avon and Somerset.

The scheme was inspired by a pilot co-ordinated by the Dutch police.

The cards contained a chemical that replicated the smell of cannabis plants when they are growing.

During the three-month campaign, launched in March, Crimestoppers received 3,553 pieces of information about cannabis cultivation from the public, which was a 28% increase from the same period last year.

Information generated by the campaign was used by the Metropolitan Police for Operation Hawk, which resulted in 34 cannabis factories closed, cannabis worth £2.1m was seized and three firearms recovered.

Merseyside Police received a 56% increase in pieces of intelligence in the first month of the campaign and at least 25 cannabis farms were identified.

Meanwhile detectives in South Yorkshire seized £89,000 worth of cannabis plants and made 15 arrests.

The results of the campaign were released as Crimestoppers published its annual impact report.

It stated that around 20 people are arrested every day as a result of information given to the crime-fighting charity, which has received 1.4 million anonymous calls and online reports since its launch 25 years ago.

Crimestoppers' chief executive, Mark Hallas, said: "Crimestoppers' commitment to fighting crime has never been stronger and we continue to be proactive and determined in exploring new and innovative ways to educate, empower and encourage the UK public to put their trust in Crimestoppers.

"Working with communities and government agencies, we believe Crimestoppers to be a truly effective crime-fighting charity that will continue to have a highly significant and successful impact on keeping communities safe."