A private team of contractors was tasked by the council to covertly rifle through the black and green bins of 250 homes in the city to discover the amounts and types of waste being thrown away.

They selected the homes to target using a database which categorises people on how posh or poor they are.

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The snoopers rummaged privately through the bins of 35 homes occupied by "wealthy achievers" and 29 from a category called "urban prosperity".

The waste was analysed from 74 so-called "comfortably off" households, 35 judged to be on "moderate means" and 77 thought to be "hard pressed". The public were not informed of the waste audit, which was carried out during the last two weeks of February 2009, because the council feared that warning households would lead to "changes in behaviour".

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Those carrying out the sampling carried letters to be handed out to any suspicious member of the public which explained what they were doing.

A Leeds council spokeswoman today stressed that the analysis was "completely anonymous" but also stressed that households should shred any confidential letters they throw out.

She said: "This information helps us to assess the how well our recycling collection schemes are working and to help us plan future waste management services.

"This is completely anonymous as no personal information is collected and no results are linked to individual households.

"We need to make sure the samples are from a representative selection of the area's population.

"We do not inform residents in advance as this could change their behaviour.

"We always advise residents not to dispose of any waste that they feel is confidential without shredding it first, as a wheeled bin on the kerbside is obviously not a secure environment."

A Freedom of Information reply revealed that Leeds council commissioned private firm Jacobs UK Ltd to carry out the waste "composition analysis."

The sample of households was selected using the "ACORN geodemographics" database, which identifies and categorises people living in certain areas, providing information on how these people generally recycle, what newspapers and magazines they may read, and what hobbies they may take part in.

It is often used by supermarkets when deciding where to build a new store.

A total of 3,200kg of green bin recyclable waste was collected from the 250 households. This gives an average weight of recyclable waste of 13kg per household per month.

Just under 59 per cent of the recyclable waste was paper or card and 13 per cent was garden waste.

Around 33 per cent of the black bin rubbish was kitchen waste, 15 per cent was paper or card and 9.6 per cent was glass.

*Properties from the following postcodes in Leeds had their rubbish secretly analysed: LS3, LS4, LS6, LS8, LS10, LS12, LS14, LS15, LS19, LS25 and LS27.