Police officers seized cannabis plants worth more than £4.2m across the Thames Valley last month - as forces nationwide stepped up efforts to tackle those trading the class B drug.

Thames Valley Police carried out 31 raids on suspected cannabis farms in June, seizing over 8,700 plants together with 28kg-worth of the packaged drug and more than £5,500 in cash.

They included a raid on a flat in Rose Hill, Oxford, where plants worth more than £100,000 were seized and a man arrested as he tried to leave the property.

Specialist operations chief Ch Supt Jim Weems said: “Cannabis factories are a very real local threat. Those tasked to look after the plants are often vulnerable and sometimes the victims of human trafficking and modern slavery.

“The size of criminal cannabis ‘factories’ means that damage is often caused to the properties themselves; the buildings can become dangerous as a result of fire risks, unlawful abstraction of electricity, fumes and water damage.”

The police activity in the Thames Valley was part of a wider operation by forces across the country.

Called Operation Mille, it involved over 1,000 warrants and searches. Police seized cannabis plants worth up to £130m and arrested 967 people for offences ranging from growing cannabis to possession of weapons.

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Steve Jupp, the National Police Chiefs’ Council lead for serious and organised crime, said: “We know that organised networks involved in cannabis production are also directly linked to an array of other serious criminality such as class A drug importation, modern slavery and wider violence and exploitation.

“This operation not only successfully disrupted a significant amount of criminal activity, but the intelligence gathered will also help inform future law enforcement across the country.

“Cannabis-related crime is often thought to be ‘low level’, however there are clear patterns around the exploitation and violence OCGs are using to protect their illicit enterprises. We also frequently find that cannabis production is just one aspect of their criminal operations and that they are complicit in wider offending which blights our communities.”

In the Thames Valley, officers made 35 arrests in connection with the cannabis-busting operation. Fifty digital devices were seized, including mobile phones, laptops and in one case an entire CCTV system.

The police asked people to be report any suspicions of a cannabis factory in their neighbourhood.

Signs to look out for included a sickly sweet smell of cannabis, the noise from industrial fans, lights being left on all night, windows being blacked out or there being condensation in the windows, they said.