BREAKING NEWS: David Cameron's closest aide arrested for child porn images.

One of David Cameron's closest aides has resigned after being arrested on child pornography allegations. Patrick Rock has been closely involved in drawing up Government policy on internet porn filters. He quit after the Prime Minister learned he was at the centre of a police probe over images of child abuse. Detectives from the National Crime Agency searched No 10 and examined IT systems and offices used by Mr Rock, deputy director of the Downing Street policy unit. 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 many years. Described as the Prime Minister's 'policy fixer', the unmarried 62-year-old had been tipped as a leading contender for a Conservative peerage only weeks ago. His arrest and resignation from Downing Street, where he has worked since 2011, will send shockwaves through the party's high command. The Prime Minister is said to be 'extremely shocked'. A friend said: 'He has known Patrick for a long time and never expected anything like this.' Mr Rock was arrested last month after officials at No 10 were made aware of a 'potential offence relating to child abuse imagery', Mr Cameron's spokesman told the Daily Mail. One well-placed source said Mr Rock had also been the subject of a sexual harassment complaint from a civil servant working in Downing Street, which is understood to have been passed to Mr Cameron's chief of staff Ed Llewellyn. 'There were accusations against him that didn't turn into a formal process. That has upset some people at No 10,' the source said. Mr Cameron's relationship with Mr Rock goes back to the Home Office, where they both worked under Michael Howard in the 1990s.