Adam Werritty is being interviewed by senior civil servants over his links to Liam Fox, amid a deepening mystery over how he managed to fund his travel for meetings with the defence secretary.
Downing St said on Tuesday morning that Cabinet Secretary Sir Gus O'Donnell would look at the "remaining questions" surrounding Liam Fox and his friendship with friend Adam Werritty.
"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," Number 10 said in a statement.
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.
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. However research done by the Times suggested Werritty earned £20,000 from his various business interests within the past four years.
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 case is how Werritty funded flights around the world worth between £25,000 and £100,000.
Fox could also 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 on Monday.