Patrick Rock resigned as deputy head of the policy unit last month
One of David Cameron's aides has been arrested on suspicion of an offence "relating to child abuse imagery", Downing Street has said.
Patrick Rock, 62, resigned as the deputy head of the policy unit on 12 February.
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 a few hours later on 13 February.
The NCA has not confirmed the arrest.
A Downing Street spokesman said that following Mr Rock's arrest, it "arranged for officers to come into Number 10 [to] have access to all IT systems and offices they considered relevant".
The prime minister was immediately informed and kept updated throughout, he said.
'Party's upper echelons'
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 was 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 and not advisers determined the policy.
BBC News - Cameron aide arrested in 'child abuse imagery' inquiry
