The Conservatives say local government officials are monitoring couples' sleeping arrangements for council tax purposes.
They have released documents they say shows that councils are invading households' privacy to check on claims for council tax discounts.
More than 7.5 million people claim a 25 per cent discount on their council tax bill because they live alone
Councils are responsible for verifying that people who claim to live alone really do so.
A "surveillance dossier" used by Rotherham Council and released under freedom of information laws has shown how claims are checked.
The document suggests officials undertake "surveillance" of cars registered to addresses "to substantiate the allegation of living together".
It also suggests surveillance "to establish if customer's partner is living at the property" and to establish if couples are living "as husband and wife."
