British Defence Secretary Liam Fox resigned on Friday over his friendship with a businessman with defence interests.
The minister has been the subject of days of speculation about his relationship with a friend, Adam Werrity, who has been passing himself off as one of the minister's advisers.
Mr Werritty had no security clearance nor government role, but met Mr Fox at least 40 times in the last 18 months at the defence ministry and abroad.
He joined the defence minister's meetings with officials, dignitaries and at least one businessman.
Questions have been raised about who paid for Mr Werritty's business activities and whether he had personally benefited from his frequent access to the defence secretary.
Mr Fox had initially dismissed claims of impropriety regarding his relationship with Werritty as baseless and "wild allegations" but later went on to apologise for allowing "distinctions to be blurred".
Cameron had given Mr Fox, 50, his support pending the findings of an inquiry, due within days, into whether he had broken ministerial rules by allowing Werritty to benefit from their friendship or have access to classified information.
The defence secretary's role is particularly sensitive given Britain's role in Afghanistan, where there are some 10,000 British troops, and in Libya, where British fighter jets have been key in a NATO campaign to protect civilians.
Liam Fox was in charge of billions of pounds of military spending, and last year launched an ambitious and controversial plan to overhaul the military to fix a £38 billion "black hole" of unfunded defence spending.