David Cameron is facing claims of a Downing Street cover-up after a senior aide was arrested on suspicion of possessing child porn.
Patrick Rock resigned quietly nearly three weeks ago after officials were made aware of a potential offence.
But Labour MPs have been asking why Number 10 only disclosed the news in response to questions from the Daily Mail on Monday.
Backbencher John Mann said it was "mysterious" that the situation had not emerged earlier.
"There has been a bodged attempt at media manipulation which is wholly unacceptable from 10 Downing Street," he said. "Of course they should have revealed it proactively."
Speaking on a visit to the West Midlands yesterday, Mr Cameron said he had been "profoundly shocked" by the allegations against Mr Rock.
"I don't think it would be right to pre-emptively brief out a criminal investigation and that's why we did not do that," he said.
"But as soon as questions were asked, as questions would inevitably be asked, we have given very full and straightforward answers, which is absolutely the right way to answer this.
"I've been clear right along and I was told about this issue pretty much as soon as it was discovered and I have been very clear we must handle this in an absolutely correct way and I am satisfied that is what No 10 Downing Street has done."
The Prime Minister's spokesman confirmed that No 10 was first made aware of the alleged offence regarding child abuse imagery on the evening of February 12.
The matter was immediately referred to the National Crime Agency and Mr Rock resigned his position as deputy head of the policy unit. In the early hours of the morning of February 13 he was arrested at his home in London.
Mr Rock has been an influential figure behind the scenes in the Conservative Party for decades, and made three unsuccessful bids to become an MP. He is reported to have been in line for a peerage.
In the 1970s, he was credited with coining the Tory slogan "Cows moo, dogs bark, Labour puts up taxes".
In 2011 he was drafted in by Mr Cameron - a fellow adviser to former home secretary Michael Howard in the 1990s - to beef up the Downing Street policy unit.
Prior to his arrest and resignation, Mr Rock had been involved in drawing up Government policy on internet porn filters.
It has emerged that the adviser was the subject of an unrelated complaint of "inappropriate behaviour" towards a female civil servant 18 months ago.
Downing Street said the complaint against Mr Rock had been dealt with "very seriously" in accordance with the Cabinet Office's human resources policy but declined to give further details.
The spokesman said the harassment allegation was "in no way related to the arrest".
"The issue was appropriately resolved with the consent of and in consultation with the individual who raised the issue," he added.