David Cameron 'shocked' by allegations against aide
David Cameron has said he feels "profoundly shocked" by allegations against a Downing Street aide arrested on suspicion of an offence "relating to child abuse imagery".
Patrick Rock, 62, resigned as deputy head of the policy unit on 12 February.
The prime minister said officials had handled the issue in "the correct way".
Number 10 said it had been made aware of a potential offence and referred the matter to the National Crime Agency, which arrested Mr Rock at his home.
This had happened on 13 February, it added.
Speaking at an event in Coventry, Mr Cameron said: "When I heard about these allegations I was profoundly shocked and I remain profoundly shocked today.
"I have to be careful about what I say about this issue because a criminal investigation is under way."
He added that he had been "told about this issue pretty much as soon as it was discovered", also saying: "I've been very clear that we must handle this in absolutely the correct way and I'm satisfied that's what Number 10 Downing Street has done,"
A Downing Street spokesman said that following Mr Rock's arrest, it had "arranged for officers to come into Number 10 [to] have access to all IT systems and offices they considered relevant".
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."
BBC political correspondent Iain Watson said Mr Rock had been a fixture in the upper echelons of the Conservative Party for three decades, initially working for Margaret Thatcher, and had been brought back into Downing Street by David Cameron in 2011.
As deputy head of the policy unit, he was one of a number of officials who had been working on policies to rid the internet of child abuse, our correspondent said.
Mr Rock was involved in preparations for a summit last year, working with the NCA, at which leading companies agreed to make it as difficult as possible to find images of abuse on their search engines.
Downing Street said ministers, not advisers, determined the policy.