Firefighters stumbled across a £30,000 cannabis factory in Oxford after neighbours saw steam pouring from the roof and thought it was smoke.

Residents in Margaret Road, Headington, thought number 11 was ablaze, but the smoke was actually hot air created by lamps used to grow cannabis plants in trays on both floors.

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.

Fire crews who found the plants inside called police, and yesterday officers cordoned off the scene and continued collecting evidence.

Pc Steve Martin, 33, based at St Aldate's police station, one of 12 officers working on the inquiry, said the cannabis that could be extracted from the 531 plants had an estimated street value of £30,000.

He added: "This was clearly a very professional operation and those involved have made a substantial investment to convert the house - they must have spent thousands of pounds.

"There were 40 heat lamps in total and the plants, which take about 13 weeks to grow, were almost ready for cultivation.

"This is clearly very lucrative for the dealers and they have been unlucky - they were caught out because the hot air from the house escaped into the cold, causing steam."

Pc Martin, who worked as a horticulturalist until two years ago when he joined the Thames Valley force, added that the gang had put curtains up around all the windows to make sure no-one saw the 140 plants in the dining area and 391 plants in two of the bedrooms upstairs.

They also drilled through the ceiling to install a large air extraction sytem.

Pc Martin said: "From the outside, this looked just like any other family house in the street but it had been rented out and abused by the tenants.

"No arrests have been made yet and we are seizing any items that will lead to the tenants' identity. We are also trying to talk to the landlord.

"Unfortunately, this practice does seem to be getting more common and we will be checking to see if there any links with previous incidents in Oxford.

"We would ask residents to be on the look-out for anything suspicious - there are tell-tale signs when these properties are being converted to grow plants.

Pc Martin said residents should look out for: * A large turnover of residents in rented properties * Curtains drawn 24 hours a day * Occupants being secretive about arriving at the home or leaving it * Steam or smoke emerging.

Neighbours said they were stunned that the home had been converted into a cannabis factory.

A 68-year-old man, who lives nearby, said: "There were some Orientals living there earlier this summer but they may have been entirely innocent. This is a nice area and you don't expect this round here."

Linda Oliver, who lives opposite the house, added: "I am totally shocked. Friends of ours used to own the house and it was a nice family home but after they sold it, the house was rented out.

"About five months ago, the house was occupied by some builders from up north but they moved out and whoever moved in after that was very quiet. The curtains were always drawn."

Anyone with information is asked to contact police on 08458 505505.