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One of David Cameron’s closest aides has been arrested on child abuse imagery allegations, it was revealed last night.

Veteran Tory adviser Patrick Rock, who has been an ally of the Prime Minister since they first began working together 20 years ago, quit before being arrested.

Rock, dubbed “the most powerful Cameron adviser most people have never heard of”, was a protege of late PM Margaret Thatcher.

He first worked with the future PM when they were advisers to Michael Howard at the Home Secretary and has held a series of high-profile posts in the Conservative Party.

The unmarried 62-year-old, who has been tipped for a peerage, was closely involved in a government group drawing up policy on internet porn filters last summer.

Mr Rock’s resignation came last month when the PM learned he was at the centre of a police probe over images of child abuse.

Detectives from the National Crime Agency searched his Number 10 office and examined Downing Street IT system.

Mr Rock, who lives in West London, resigned hours before being arrested, Number 10 insiders said.

His shock arrest is a massive blow to Mr Cameron and has sent shockwaves through the Tory party.

Last night, a Number 10 spokesman said: “On the evening of February 12th, Downing Street was first made aware of a potential offence relating to child abuse imagery.

“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 13th, a few hours after Downing Street had reported the matter.

“Subsequently, we arranged for officers to come into Number 10 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.”

Mr Cameron is said to be ‘extremely shocked’. A friend of the PM last night said: “He has known Patrick for a long time and never expected anything like this.”

One well-placed source said Rock had also been the subject of a sexual harassment complaint from a civil servant working in Downing Street, which had 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.

Downing Street refused to comment on the allegation of sexual harassment, but said: “All complaints are dealt with very seriously and are dealt with appropriately.”

As Deputy Director of the Downing Street policy unit, Rock was involved in the PM’s flagship plan to put porn filters on home computers.

He was brought back to Government as Mr Cameron’s policy “fixer” almost three years ago and had been tipped for a peerage.

A Number 10 document published last year said: “The Prime Minister is due to make Rock a peer, in recognition of his long service and the trust that he puts in his judgement.”

Rock, who studied Modern History at Oxford University, worked in Tory Central Office in the John Major years and worked as a Special Adviser for Michael Howard in the Home Office alongside David Cameron.

He was given a free-ranging role in the policy unit trouble-shooting across a range of policy and political issues.

Mr Rock, who has repeatedly tried to become an MP, is known in Westminster for coining the phrase “Cows moo, dogs bark, Labour puts up taxes” used by the Tories in the 1970s.

He narrowly missed out on becoming an MP when he lost out to Portsmouth South LibDem MP Mike Hancock in 1984.

In January last year Rock committed a major blunder when he was photographed carrying a document branded by labour leader Ed Miliband as an ‘audit of coalition broken promises’.

The document was a discussion of the pros and cons of releasing a review of which targets the Coalition had missed and which it had met alongside the official Mid-Term review.

Rock was pictured carrying the document in Downing Street having failed to cover it up and leaving it in the full view of photographers.