A top aide to British Prime Minister David Cameron has been arrested as part of an investigation into “child abuse imagery”, Downing Street confirmed on Tuesday.

Patrick Rock resigned as deputy head of Downing Street’s policy unit hours before he was arrested three weeks ago.

“On the evening of February 12 Downing Street was first made aware of a potential offence relating to child abuse imagery,” the Premier’s office said in a statement.

“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 Number 10 and have access to all IT systems and offices they considered relevant.”

Rock worked with Mr. Cameron at the Home Office in the 1990s, according to the Daily Mail, the newspaper which broke the story.

“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 statement added.

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