Patrick Rock, one of David Cameron’s senior aides, was arrested last month on child pornography allegations.
Rock, 62, resigned as policy adviser after being questioned by the police.
Rock, who was involved in drawing up internet porn filters for the UK, is known to be a close aide for Cameron for two decades.
A Downing Street spokesman said: “On the evening of February 12th Downing Street was first made aware of a potential offence relating to child abuse imagery. It was immediately referred to the National Crime Agency (CEOP). The Prime Minister was immediately informed and kept updated throughout.
“Patrick Rock was arrested at his home in the early hours of February 13th, a few hours after Downing Street had reported the matter.
“Subsequently, we arranged for officers to come into Number 10 have access to all IT systems and offices they considered relevant.”
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