Few will have heard of Patrick Rock before now - except for those in the Tory Party's high command.
At 62, he has had a political career spanning four decades, working with senior party figures such as the former Tory leader Lord Howard, and the former party chairman, Lord Patten.
When the Prime Minister told me today he was "profoundly shocked" to hear of the allegations against Mr Rock involving images of child pornography, that was probably an understatement.
Mr Rock has been a confidant and adviser to David Cameron since the 1990s. The Prime Minister brought him back into the heart of government in 2011 as number two in the policy unit, where Mr Rock worked on issues which included internet filters to protect against child abuse images online.
David Cameron said he had to be careful what he said today because this was an ongoing criminal investigation.
But there was no disguising the fact that - whatever the outcome of the investigation - Mr Rock's arrest and resignation are a serious blow, both personally and professionally.
ITV News Political Correspondent Carl Dinnen has tweeted:
Asked if PM knew of earlier complaint against Rock, PMOS said 'The PM is always kept informed of relevant staffing issues.'
The Prime Minister's spokesman has said a complaint of inappropriate behaviour made against adviser Patrick Rock during his time at Downing Street was not linked to his arrest over alleged child abuse images.
ITV News Political Correspondent Carl Dinnen tweeted:
BREAKING; PM's spokesman says complaint of inappropriate behaviour against Patrick Rock was not linked to his arrest.
PM's spokesman says Rock complaint was resolved 'with the consent of and in consultation with the individual who raised the issue'
The Prime Minister has insisted he has handled the arrest of advisor Patrick Rock over alleged child abuse images in "an absolutely correct way."
Answering a question from ITV News political correspondent Libby Wiener, David Cameron said: "Obviously when I heard these allegations I was profoundly shocked and I remain profoundly shocked today.
"In terms of the release of this information, I don't think it would be right to preemptively brief out a criminal investigation and that's why we did not do that but as soon as questions were asked, as questions would inevitably be asked, we have given very full and straightforward answers.
"I was told about this issue pretty much as soon as it was discovered. I've been very clear that we must handle this in an absolutely correct way and I'm satisfied that that is what Number 10 Downing Street has done."
Downing Street has confirmed Patrick Rock was one of the advisers involved in the Government's policy on internet filters to protect against child abuse images online.
Leading search engine companies Google and Microsoft agreed in November to introduce changes that will prevent such images from being listed in results for more than 100,000 searches.
A Downing Street spokeswoman said, "Patrick Rock was one of a number of advisers and officials involved in dealing with this issue but the work was led by somebody else, and decisions were taken by ministers."
The 62-year-old adviser was brought back into Downing Street by David Cameron in 2011 after they both worked in the Home Office under Michael Howard in the 1990s.
Deputy Political Editor Chris Ship reports that Downing Street have said policy adviser Patrick Rock resigned from his job on the day Downing Street was first made aware of a potential offence relating to child abuse imagery. He was arrested the next day.
No10 clarify that policy adviser Patrick Rock - arrested over allegations of child abuse images - resigned from his job on February 12
Downing Street has confirmed that a Number 10 policy aide has been arrested on suspicion of an offence related to child abuse images.
In a statement, it has 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.
Downing Street has confirmed that a Number 10 aide has been arrested on suspicion of an offence related to child abuse images.
Patrick Rock was arrested on Thursday 13 February after Downing Street reported the matter to the police.
It added that the Prime Minister "was immediately informed and kept updated throughout."
Number 10 said it would not pre-empt the discussions being held in Northern Ireland today and refused to say what timetable it was working to.
The Prime Minister's official spokesman said:
When serious concerns are raised, it's a matter of urgency to consider these things but discussions are ongoing. I'm not going to try and pre-judge how those will pan out.
Clearly, all the issues that the First Minister raises will, of course, be looked at.
He refused to be drawn on when the Prime Minister was aware of the 38 letters sent out to IRA suspects since the Coalition Government took power.
The Metropolitan Police helicopter captured this picture of the Downing Street Christmas tree: