A landlord duped into renting her Oxford house out to cannabis farmers posing as students yesterday warned other homeowners to be on their guard.

The 38-year-old mother-of-two went to inspect the house in Harcourt Terrace, Headington, at 8pm last Thursday and discovered more than 200 cannabis plants being cultivated.

But the "Brookes students" - three Chinese men and a woman aged about 18 to 25 - had left, and they have not yet been traced by Thames Valley Police.

It is the latest in a spate of cannabis factories uncovered across the city in the past 18 months. The landlord told the Oxford Mail the four bedrooms in the house had been converted for growing the plants and holes had been cut in the ceiling to accommodate large metal pipes used to remove condensation. Plastic sheeting was also laid down to retain heat.

She said the tenants moved into the £315,000 house in March and paid the rent as expected.

"They seemed nice and provided references, but they told me they did not need any beds or other furniture because they had been living in the country for a few years and had their own," the landlord said.

"In future, I will ask to see passports and identification papers. I could have checked with Brookes to see if they were really students.

"Landlords need to take all the precautions they possibly can with their tenants and get access to their properties regularly to make sure the houses are not being abused."

She added: "It will cost me thousands to repair the damage - I don't know how much - and I am losing about £1,500 a month rent while the builders are in."

The landlord said she became suspicious when she noticed that the curtains were drawn every time she drove past the house.

Southern Electric made sure the house was safe before police seized the plants.

Officers have praised her for acting swiftly after suspecting her property was being used and have urged other landlords to vet tenants carefully, checking all possible references.

It was the 17th drug factory found in the city so far this year. If all the cannabis had been harvested, the drugs would have had a street value of more than £7m.