Police have 'sniffed out' a huge cannabis factory - next door to their own headquarters.

The plants were uncovered in an 'empty' factory unit after officers became suspicious about the smell just yards from the main custody suite at South Wales Police station.

Three men were arrested at the scene in Bridgend, as officers seized the marijuana plants.

Neighbouring business owners claimed officers' had described the drugs haul as the 'biggest they had ever found.'

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Police 'sniffed out' the large cannabis factory right next door to the South Wales Police HQ in Bridgend

The factory was just yards from the main custody suite at the South Wales Police HQ

'No-one had a clue, but it could have been going on for a number of years,' said car maintenance worker Hayley Pells, 33.

'They appear to have been very professional. They invested heavily in being very secure and very discreet.

'For a long time we thought it was empty. We didn't see anybody coming or going for extremely long periods of time. This isn't unusual as lots of the units are used only for storage.

'They (the police) told us they were suspicious about the unit and sent a helicopter over it to scan the temperature but it was no different to any of the others.

'They were still suspicious about the unit and when they noticed a smell from it they acted.

'They told us that the plants were growing in insulated boxes which was why nothing showed up on thermal imaging.'

Police officers seized the marijuana plants as they launched an investigation in the large warehouse unit

Scenes of crime officers, wearing white boiler suits, were seen entering the factory building

Her father Andy Murdock, 63, said he had been told by officers at the scene it was the biggest haul of cannabis they had ever found.

'Our unit is 3,000 sq. foot and they had a double unit and a single unit and upstairs as well. It could be as much as 9,000 sq. foot of weed.

'It's amazing if you think about it and really brave of them to grow it next door to the police.

'I'm really not sure how they thought they would get away with it. Literally you could throw a brick from the back of this unit and it would land in the police station it's that close.

'We had no idea whatsoever what was going on. I'm shocked and very surprised.'

A South Wales Police spokeswoman said: 'Three men have been arrested and are currently in police custody.