Eastern European modern-day slaves have been collecting clothing donated by the public to charities, police believe.

Officers from Northumbria police swooped on a number of addresses, rescuing a dozen alleged slaves, and arresting six people in Newcastle and Gateshead in connection with the case this Monday.

A “Lithuanian organised crime group” is understood to be operating in the area, the force said in a statement, using a factory in North Shields and trafficking men and women from Eastern Europe to work in the region.

Eastern European gangs are known to collect donated clothing before legitimate charities or set up fake charities to steal the garments. In this case, the gang was contracted by genuine, unnamed organisations and it is the first case involving slavery.

The migrant gang is thought to have brought the clothes back to their factory, where the garments were sorted before being sold overseas. A cut of the profit was handed to the charity.

The Charity Retail Association, which represents the interests of charity retailers, said there is a large problem with groups taking a cut of profits from charities and selling donated clothes abroad, as well as organising “bogus collections” and “selling the clothing for their own profit”.

Superintendent Steve Barron, who led the operation, insisted police do not believe any of the charities involved knew that those collecting their bags were potential victims of modern-day slavery and human trafficking.

He said: “Often individuals don’t realise that they are victims and the small wage they earn in this country often exceeds anything they would earn in their home country.

“They are brought into the country on the promise of work, housed in sub-standard accommodation and their benefits and finances are all controlled by their employer.”

HMRC is also running a separate investigation to establish whether any of the proceeds from the illegal business are going to the charities in question.

Northumbria Police and Crime Commissioner Dame Vera Baird QC added: “I am pleased to see great moves forward in Northumbria’s response and understanding of modern-day slavery through targeted operations like this and 2017’s Op Kestrel, which saw significant success.”