The operation saw police confiscating “just over six tonnes of marijuana, among which were 3.5 tonnes of buds in vacuum-packed thermal bags,” the National Police and Guardia Civil police said in a joint statement.

They described the haul as “the biggest quantity of marijuana ever seized in Spain”.

The investigation began in March 2021 when investigators discovered a 32-hectare (80-acre) plot in Vilassar de Mar, some 30 kilometres (18 miles) up the coast from Barcelona which was being used for growing hemp, or cannabis plants, on an industrial scale.

But the signs put up around the site suggesting it had a legal purpose were false, with police quickly realising it was being used for the large scale cultivation of marijuana.

Thousands of plants were being grown in different warehouses and greenhouses, all carefully monitored by more than 100 cameras, police said.

The site, which was also illegally tapping into the power grid, had been operational since at least 2018.

During the operation, police arrested five people, who were charged with crimes against public health and electricity fraud. They also found 17 kilos of hashish resin, which the gang were starting to produce.

After decades as the gateway for Moroccan hashish to enter Europe, Spain is now seeing illegal plantations multiply as the country morphs into a cannabis production hub, attracting criminal gangs from across the continent.