Bristol’s biggest ever cannabis factory has been uncovered – in Bedminster.

The former NatWest bank on East Street had been transformed into an illegal enterprise growing as many as 2,000 plants, which police believe could have produced drugs worth well in excess of £1 million on the city’s streets.

Hundreds of people would have walked past the seemingly-empty building every day, not knowing what was going on behind the heavy brown front doors at number 31.

Acting on a tip off from the Metropolitan Police, officers from South Bristol Drugs Team used a battering ram to get into the building at about 1.15am yesterday.

Inside, cannabis plants were being grown in 10 different rooms – from saplings carpeting the floor to mature plants with flowering heads more than 3ft high.

Even the former bank vault – where once thousands of pounds would have been kept – was now home to thousands of pounds worth of drugs.

As officers went from room to room they found two men, believed to be Vietnamese, hiding among the greenery.

They were arrested on suspicion of cultivating cannabis and were being questioned last night.

Sergeant Jon Bane and the drugs team have carried out scores of raids over the last few years but he believes this one is the biggest haul yet.

“We’ve only counted what we’ve found in three growing rooms so far and we’ve already found more than 700 cannabis plants,” he said.

“This is a large scale sophisticated set up capable of producing many thousands of plants. When the plants are fully grown they are worth about £1,000.

“We’re looking at as many as 2,000 in total, which is the biggest job we’ve ever done and even bigger than the one our Atrium team carried out in Speedwell last year.”

In November, 1,200-1,400 plants were found in an industrial unit on Deep Pit Road.

Mr Bane added: “It is quite unusual to find an operation of this magnitude in a building like this, on such a busy street. They normally go for a bit more seclusion.

“Small, one-bedroom cannabis growing factories are common in South Bristol at the moment, but not ones like this.

“It looks as though most of the growing rooms have been set up towards the rear of the building so that people in the street were less likely to smell the cannabis.”

In the main ground-floor room, where once customers would have queued to see bank cashiers, there were about 40 bags of soil and more than 100 plastic pots of varying sizes ready to be used for the next crop.

Flagons of liquid fertiliser lined the corridor leading to a small room being used as a kitchen and to grow saplings. The peeling back of curtains revealed hundreds more plants in rooms in the cellar and on the first and second floors.

Each room had plants of different maturity, showing the ordered and organised way in which the enterprise had been set up.

Scores of plugs and wires sprouted out from many a wall to power the heat giving lights that are vital for the plants’ rapid growth.

Cables had been used to bypass the electricity meter, which would not have been regularly checked as the building had been thought to be unoccupied.

Electricians from Western Power made the electrics safe yesterday afternoon as officers began to tally up what had been found and forensic officers gathered evidence.

But among the plants and paraphernalia there were numerous signs that the building had been lived in for months, including blankets, magazines, mobile phones, food and drink containers, chop-sticks and even underwear hanging up to dry.

Mr Bane said it was not yet known who owned the building, which has not been used for at least five years, and that keys found inside the front door were believed to have given the two men arrested regular access.

Alex Neil, who runs the International Cafe next door, said: “I’m very surprised at what the police have found. We had no idea anything like this was going on. About six months ago there were problems with the drains and work had to be done, but we are very shocked.”

Last year, South Bristol Drugs Team and neighbourhood police executed 254 drug warrants, recovering a total of £2,408,776 worth of drugs and £67,027 in cash.

Mr Bane added: “This operation should send a strong message that drugs will not be tolerated. I would urge the communities to continue to help us by reporting any information they have.

“I assure you that it will be met with a positive police response and we will continue to be relentless in our pursuit of drug dealers.”

To report drug dealers ring 0845 456 7000 or Crimestoppers, anonymously, on 0800 555 111.