Civil liberties campaigners have warned that using spy planes to search for illegal 'beds in sheds' developments could be seen as "over the top".

Harrow Borough Council spent around £20,000 on the aircraft to detect cannabis factories, homes without proper insulation and so-called 'beds in sheds' - garages or other outbuildings used as flats without authorisation.

Thermal images revealed 319 outbuildings could be being rented out illegally,  often to "exploited" immigrants paying reduced rent.

But some have voiced fears the authority could have used the aircraft as a means of snooping on its residents.

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Nick Pickles, director of civil liberties group Big Brother Watch, said the council should be "fully transparent" about such exercises.

He said: "They need to make sure they do not snoop on people who are not under any suspicion of wrongdoing.

"People have no idea what information is being recorded by these spy planes or the legal framework it falls under. This needs to be urgently addressed by Harrow Council.

"People are right to wonder if this is a very over the top response that will lead to people who have done nothing wrong being mistakenly targeted."

Independent councillor Husain Akthar said: "We don’t want any unwanted characters in Harrow, but we need to look at the best way to approach this.

"Peering into homes could be crossing the boundaries of interfering with privacy. They should do all they can to crackdown on beds in sheds, but I wonder if this is a sneaky response."

Despite this, in a Harrow Times poll, 76 per cent of people supported the use of the planes. Another 19 per cent disagreed, and 5 per cent said they did not know.

Slough, in Berkshire, used the same technology in 2013 to find possible illegal housing.

Before the operation, planning officers estimated there were 75 cases of unauthorised 'beds in sheds' in Harrow - a quarter of the suspected cases uncovered by the plane.

Officials will now visit the properties identified and will compare the images with other records to establish if there are more.

And Tory council leader Susan Hall insisted the plane was an "information seeking" device.

She added: "It’s absolute rubbish to call it a spy plane. It has given us incredible information and has been very useful.

"As well as detecting beds in sheds and people being exploited, we have been able to help many elderly people who do not have proper insulation in their homes, and stop cannabis factories."

She pointed out the "hidden community" living in illegal flats drew on services for which the council received no extra funding, and were themselves at the bottowm of a "pyramid of exploitation".

She added: "They can say what they like, but this has been nothing but a good thing for our taxpaying residents."