The sex lives of council-tax payers are being secretly monitored by local authority inspectors to establish whether residents claiming single person’s discounts are really living alone.

Undercover snoopers are being used to find out how often lovers visit and whether supposedly single residents are sharing a bed every night with the same person.

The covert tactics have been condemned as disproportionate use of council powers. Methods include inspectors parking discreetly outside homes to monitor lovers’ movements and observe whether the same person leaves the house in the morning and returns at night. They also check on how often lovers’ cars are parked outside houses and even whether women are sleeping with their landlords.

A surveillance dossier used by the Labour-run Rotherham council shows permission has been given to inspectors to use “drive-past surveillance”, sweeping round and round the block for a glimpse of lovers staying overnight.

Local authorities have adopted the techniques after the government urged them to carry out “spot checks” on properties where a single-person council-tax discount is claimed. Councils are also demanding that householders give access to their bedrooms in return for the single-person discount. Inspectors can use the searches to check bedrooms for evidence of live-in lovers not disclosed to the authorities.

Claimants of a 25% discount for single people in Thurrock, Essex, have to sign a form authorising “the council or its agents to make inquiries to corroborate this claim” and “inspect the property”.

The Conservatives have accused Labour of wanting to check people’s bedrooms for underwear and creating a “surveillance state where spying on citizens becomes the norm”. They said it was wrong for people to be spied on and interrogated about their sex lives to qualify for discounts.

“Councils will naturally wish to ensure council-tax discounts and benefits are not wrongly claimed, but I am concerned that innocent citizens will be spied on through heavy-handed and disproportionate use of town-hall snoopers,” said Eric Pickles, the Tory local government spokesman.

“There are far less intrusive and more cost-effective ways of vetting council tax than paying town-hall officials to camp overnight outside people’s homes.”

Rotherham council said its covert operations were legal and that “the intrusiveness of the surveillance is kept to a minimum, only doing the least amount necessary to obtain the information required”.

The Department of Communities and Local Government said: “Everyone would agree councils should take measures to ensure those people who claim the single-person discount on their council tax are eligible to do so. It is up to individual councils to appropriately use the powers that they have to do this.”

Times Recommends

• Baroness Ashton: EU couldn’t make it up
• The other belles de nos jours
• Why too much of a good thing is bad for you