A senior adviser to David Cameron was arrested on child abuse images allegations, Downing Street said tonight.

Patrick Rock, who had been involved in drawing up proposals for internet porn filters, resigned after being questioned by police. Officers from the National Crime Agency examined Downing Street’s computer systems and offices used by Mr Rock, 62.

He was an been adviser to the Conservative party for 30 years and got to know Mr Cameron in the mid-1990s when they were both worked for Michael Howard when he was Home Secretary.

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The Prime Minister brought him into Downing Street in 2011 to work in the Number 10 policy unit. He took responsibility for home affairs issues and was among officials who were involved in drawing up controls against internet images of child abuse.

A Downing Street spokesman said: “On the evening of February 12, Downing Street was first made aware of a potential offence relating to child abuse imagery. It was immediately referred to the National Crime Agency.

"The Prime Minister was immediately informed and kept updated throughout. Patrick Rock was arrested at his home in the early hours of February 13, a few hours after Downing Street had reported the matter.

"Subsequently, we arranged for officers to come into No 10 and have access to all IT systems and offices they considered relevant.

"This is an ongoing investigation so it would not be appropriate to comment further, but the Prime Minister believes that child abuse imagery is abhorrent and that anyone involved with it should be properly dealt with under the law.”

The Daily Mail quoted a friend of the Prime Minister as saying: “He has known Patrick for a long time and never expected anything like this.”