Downing Street has confirmed Patrick Rock was one of the advisers involved in the Government's policy on internet filters to protect against child abuse images online.
Leading search engine companies Google and Microsoft agreed in November to introduce changes that will prevent such images from being listed in results for more than 100,000 searches.
A Downing Street spokeswoman said, "Patrick Rock was one of a number of advisers and officials involved in dealing with this issue but the work was led by somebody else, and decisions were taken by ministers."
The 62-year-old adviser was brought back into Downing Street by David Cameron in 2011 after they both worked in the Home Office under Michael Howard in the 1990s.
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