Three people have been arrested in east London as part of a police probe into a group suspected of using forced labour to produce cannabis.
A man in his thirties and a woman in her twenties were arrested on Thursday at a property in Romford.
They were held on suspicion of production of cannabis, holding a person in slavery or servitude, and possession of Class A and B drugs.
Another man was arrested at the same address on suspicion of immigration offences and remains in custody.
Two more men, one in his twenties and another in his thirties, were arrested at a property in central Birmingham on suspicion of production of Class B drugs, possession of Class B drugs and immigration offences.
Officers from the National Crime Agency also conducted searches on two properties in Burton-on-Trent, Staffordshire, as part of their investigation.
Four suspects were released under investigation, while one remains in custody pending immigration enquiries, said police.
NCA operations manager Neil Gardner said: “This is a complex investigation focused on a crime group believed to be using forced labour to produce cannabis.
“We know organised crime gangs often exploit the vulnerable to further their profits, both in the production of illegal drugs and in their distribution.
“In some cases we have seen victims threatened and suffering extreme violence as the criminals exert control. This is why tackling modern slavery is a high priority for the NCA.”
In a separate unconnected investigation by the agency, a couple who kept vulnerable people as slaves were jailed on Wednesday.
Marcos Tancos and Joanna Gomulska, both 46, and of Brentry Lane, Bristol, were the ringleaders of a modern slavery and human trafficking operation in Bristol.
Tancos was sentenced to 16 years and Gomulska to nine after being convicted of keeping victims as slaves and making them work at a car wash.
Officers urged the public to contact the 24/7 Modern Slavery Helpline on 0800 0121 700, or your local police on 101 if they suspected anyone to be a victim of modern slavery.