They followed their noses - and sniffed out the biggest cannabis factory ever found on Oxford's Blackbird Leys estate.

Pc Alex Shepherd and Pc Phil Gardiner, of the Blackbird Leys proactive team, were carrying out routine patrols on Saturday morning when they grew suspicious about a house in Balfour Road.

They knocked at the door and, when there was no answer, forced entry and found 250 fully mature cannabis plants in three rooms in the house, as well as the loft.

Pc Shepherd, 22, who is based at Cowley, said: "We were on routine investigations for an unrelated matter when we smelled the cannabis. We recognised a few other signs from past experiences of finding these things and realised this could be a cannabis factory.

"We then gained entry to the house and our suspicions were confirmed.

"There were 427 plants in the house and this is the biggest find we have had in Blackbird Leys to date, and possibly in Oxford."

Pc Shepherd said they were pleased with the discovery, particularly since Blackbird Leys residents have highlighted drugs as one of their biggest concerns on the estate.

He said: "Finding this is very significant for the community of Blackbird Leys. They have highlighted that drugs are a problem and we know we need to combat this.

"There have been about 10 discoveries of cannabis factories so far this year."

Police say the number of cannabis factories are on the increase and are appealing to residents to alert them if they see or hear anything suspicious.

Neighbours should look out for: a large turnover of residents in rented properties; curtains drawn 24 hours a day; occupants being secretive about arriving or leaving the property; steam or smoke emerging from it.

No-one has been arrested in connection with the discovery at the weekend.

Anyone with information is asked to call Thames Valley Police on 0845 8505 505 or Crimestoppers on 0800 555111.