A close aide to David Cameron has resigned over allegations involving child abuse images, Downing Street has said.
Patrick Rock has been closely involved in drawing up government policy on internet porn filters.
He was arrested by detectives from the National Crime Agency (NCA) over a "potential offence relating to child abuse imagery", a Downing Street spokeswoman said.
The spokesman said: "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."
Mr Rock was a protege of Margaret Thatcher and has held a series of senior posts in the Conservative Party. He has been close to Mr Cameron for two decades.
The 62-year-old was arrested at his home on February 13 "a few hours" after Downing Street contacted police and officers were given access to all IT systems and offices at Number 10, she added.
An NCA spokeswoman declined to comment, stating it did not name suspects on arrest.