The Cabinet Secretary Sir Gus O'Donnell will look at the "remaining questions" surrounding Liam Fox and his friendship with friend Adam Werritty.
Questions remain about the defence secretary's dealings with Werritty, despite a strong performance by the defence secretary in the House of Commons on Monday.
Number 10 said on Tuesday: "The Cabinet Secretary is looking at all of these questions. The Cabinet Secretary is addressing the remaining questions. There are a number of questions people have been asking and we want to answer those questions."
Werritty is thought to be being interviewed by senior civil servants.
Fox has apologised to the prime minister and the Commons for allowing his private life and professional life to become "blurred" and received strong support from many on the government backbenches.
But following his statement to MPs, the Ministry of Defence released a list of the 18 overseas visits Fox made where Werritty had been present, as well as listing the dates of 22 times Fox's best man visited him at the ministry.
The timing of the publication meant MPs were unable to question Fox on the details of the trips.
While David Cameron has given his backing to his under fire defence secretary, Sir Gus O'Donnell, the country's top civil servant, has launched a full inquiry into Fox's association with Werritty.
Werritty met Fox in including Dubai, Hong Kong, Singapore and perhaps most controversially joined the defence secretary for a steak dinner with the incoming commander of the Isaf force in Afghanistan, General John Allen.
Central to the question is how Werritty funded an estimated £100,000 worth of flights around the world.
Fox could be in trouble if Werritty, a defence lobbyist, was found to have profited from the trips, his access to Fox and his knowledge of the defence secretary's diary.
Sensing blood, Labour's shadow defence secretary Jim Murphy said Fox had "driven a coach and horses through the rules". He urged Cameron to refer Fox to the independent adviser on ministers' interests, Sir Philip Mawer.Suggest a correction