The CIA once implanted microphones into a cat in a bizarre attempt to spy on Soviet Russia.

The cat had a microphone, antenna and battery pack surgically embedded into its skin so the feline could act as a covert recording device.

Recently declassified documents show how the scientists responsible for the cruel research were praised by spy chiefs for their 'pioneering' work.

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The strange eavesdropping technique, though never used in the field, resurfaced this week when WikiLeaks tweeted a link to declassified CIA memos, which were first released in 2001

PROJECT ACOUSTIC KITTY - Ran from 1962-1967 - Cost $13 million (£10 million) - A cat's tail was used as an antenna - A wire ran up its spine to a microphone in its ear - The device's battery pack was sewn into the animal's chest - The final 1967 report on the project concluded it was impractical Advertisement

The strange eavesdropping technique, though never used in the field, resurfaced this week when WikiLeaks tweeted a link to declassified CIA memos, first released in 2001.

The research was dubbed 'Project Acoustic Kitty' and cost $13 million (£10 million) over its five-year development in the 1960s.

The cat's tail was used as an antenna with a wire travelling all the way up its spine to a microphone in the animal's ear.

The equipment's battery pack was sewn into the cat's chest.

Victor Marchetti, a former CIA officer, told The Telegraph that year of the gruesome creation.

He said: 'They slit the cat open, put batteries in him, wired him up'.

The CIA once implanted microphones into a cat in a bizarre attempt to spy on Soviet Russia. Declassified documents (pictured) reveal that the work was praised by CIA chiefs as a 'remarkable scientific achievement'

'They made a monstrosity. They tested him and tested him.

'They found he would walk off the job when he got hungry, so they put another wire in to override that,' he added.

The final 1967 report on the project concluded it was non-practical, signalling the end of the research.

But the memo hailed the 'remarkable scientific achievement' reached by the American spy agency.

'The work done on this problem over the years reflects great credit on the personnel who guided it,' the document concluded.