The National Fraud Intelligence Bureau has set up a team to analyse charity bag collection fraud.
Detective Superintendent Steve Head, head of the economic crime directorate at the City of London Police, which runs the government-funded bureau, said the team would help police develop a national strategy to tackle the crime.
The development follows a round-table meeting on the issue in January, attended by charities and Nick Hurd, the Minister for Civil Society.
Head told Third Sector three police officers were already working in the team, which would gather and analyse intelligence from around the country and develop a more joined-up approach to fraud.
He encouraged charities to send employees on voluntary, part-time secondments to help them. The clothing collection company Clothes Aid had already pledged to send two people, he said.
"If I had one message for the sector it would be 'please engage with us on this'," he said. "We need people who understand better than we do how it all works.
"We believe this criminality is a very serious issue. There’s potentially a national or even international element to it."
Head said police hoped to publish an assessment of how widespread the problem was, but there was no timeframe for doing this.
He said City of London Police was carrying out two national investigations into charity clothing fraud.
Common aspects of bag crime are fraudsters soliciting collections for bogus charities or using the names of genuine charities, and the theft of bags left on doorsteps for genuine charities.
Reliable estimates of the cost to charities of bag fraud are difficult to obtain. The Charity Retail Association says the figure is at least £2.5m a year, and the British Heart Foundation has said its losses alone are £3m a year.