Redfaced CIA spooks have admitted using an illegal snooper cookie to tail people who visited its website.

The cookie informed the CIA about all the websites users visited for years after it was installed on a PC, despite the fact that such persistent cookies are banned by the US government.

The CIA website claims that it does not use the cookies to gather and store information about visits to its sites.

But the snooper software was found by Daniel Brandt, president of Public Information Research, who explained that he picked up the cookie because he was in a CIA internet reading room searching for pages with particular 'security sensitive' words.

He believed that the CIA was using the cookie to keep tabs on those whose interest in sensitive information could identify them as security risks. The cookie would send web browser information to the CIA every time the user accessed the internet.

The CIA admitted having the cookie installed on the site and removed it the instant it was made public.

A spokesman maintained that the cookie's presence was "a mistake, not intentional". An outside company had redesigned the reading room website and installed the software for web analysis.