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Police are hunting an audacious drug dealer who set up a secret cannabis factory at a disused building in the grounds of a city hospital.

An operation set up to cultivate 30 illegal plants had been working, possibly for many months, at Whitchurch Hospital, Cardiff.

Police only stumbled upon the hidden factory in the psychiatric hospital’s rambling grounds during a search for a missing person believed in the area on June 30.

They entered an unused ward in the main hospital building, which had been boarded up and isolated for decades, and found an Aladdin’s cave dedicated to the illicit crop.

And it has emerged, that in an extremely bold move, the dealer or gang had tapped into the hospital’s electrical supply and were using it to power ultra violet lighting equipment and ventilators.

Hospital bosses have insisted that the cannabis factory was found in a “sealed and unused” section of the main building and that security at the site is robust.

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An investigation has been under way for six weeks focusing on who exactly had the inside knowledge and bare-faced cheek to set up and run the cannabis cultivation plant.

What is clear is that those responsible must have been aware that the building represented an ideal secret location for the illegal operation.

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Overlooked and apparently never checked, it was effectively hidden in plain view with its crucial electrical supply still working.

While security guards at the hospital make regular foot patrols in the area, it is understood they never enter any of the supposedly locked, boarded-up and disused buildings on the site.

Police seized all the drug making “paraphernalia” that they discovered and have also seized and destroyed the cannabis plants, retaining only samples for use in any future prosecution.

A forensic team has also carried out a detailed investigation of the scene as part of the ongoing search for DNA, fingerprint and other evidence.

“South Wales Police can confirm that a cannabis factory of around 30 plants was discovered within the grounds of Whitchurch Hospital in Cardiff,” a spokesman said.

“The discovery was made on June 30 as part of unrelated police activity in the area. The matter is currently under investigation.”

Cardiff and Vale University Health Board runs the hospital site.

Abigail Harris, the health board’s director of planning, said: “We would like to reassure the public that the health board’s security teams carry out regular patrols of all our sites.

“This has happened in a small corner of a big hospital in an area that was sealed and unused for many, many years. It is a highly unusual and isolated case which is being carefully reviewed with police.

“The health board is doing all we can to support the investigation and find those responsible.

“We would encourage anyone with information about this incident to contact the police.

“As this is an ongoing police investigation it would be inappropriate to comment further at this time.”