A senior aide to British Prime Minister David Cameron has resigned after being arrested on suspicion of child pornography offenses, Downing Street confirmed Monday.

Patrick Rock, 62, was arrested by officers from the National Crime Agency last month.

“On the evening of February 12, Downing Street was first made aware of a potential offence relating to child abuse imagery,” said a Downing Street spokesman.

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It was immediately referred to the National Crime Agency, the spokesman said.

“Patrick Rock was arrested at his home in the early hours of February 13, a few hours after Downing Street had reported the matter,” the spokesman said.

“The Prime Minister was immediately informed and kept updated throughout,” he added. “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.”

Rock, who helped draw up guidelines on Internet porn filters, was an adviser to the Conservative party for 30 years.

A friend of Cameron told the Daily Mail, which uncovered the arrest, that the prime minister was “extremely shocked.”