A close aide to British Prime Minister David Cameron has resigned over allegations involving child abuse images, Downing Street said.
Patrick Rock, a special adviser on policy, was arrested by detectives from the National Crime Agency (NCA) over a "potential offence relating to child abuse imagery", a Downing Street spokeswoman said.
He was arrested at his home on 13 February and officers were given access to all IT systems and offices at Number 10, she added.
The spokeswoman confirmed Mr Rock, who was deputy head of Downing Street's policy unit, resigned on 12 February.
In a statement, Downing Street said it was first made aware of a potential offence on the evening of 12 February.
"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 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.
"This is an on-going 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."