Liam Fox has ordered a close friend to stop handing out business cards naming himself as an adviser to the Defence Secretary.
Dr Fox made the revelation as he asked a senior Whitehall official to investigate "wild allegations" about his relationship with Adam Werritty.
In recent days the Defence Secretary has been accused of putting national security at risk by allowing Mr Werritty - a former flatmate who was best man at his wedding - access to the Ministry of Defence 14 times in 16 months despite not being a government employee.
Questions have also been asked after Mr Werritty joined Dr Fox at an event in Sri Lanka which was part of an official visit to the country.
Dr Fox said he had asked his most senior civil servant Ursula Brennan to investigate whether the Ministerial Code had been breached, vowing to "stand by" the findings.
During a round of television interviews, the 50-year-old also said he had asked Mr Werritty to stop giving out businesses cards bearing Westminster's portcullis logo and listing him as an "advisor to Rt Hon Dr Liam Fox MP" - one of which was pictured in The Guardian today.
Aides said the request had been made a "few months" ago.
"I have made it very clear to him that it is unacceptable to carry a card saying that he is a personal adviser," Dr Fox said. "I understand those cards are no longer used."
Shadow defence secretary Jim Murphy welcomed the investigation but said it "seems like a panic
measure".
"The secretary of state has ordered an inquiry into himself in order to prevent his top civil servant forcing one upon him," Mr Murphy said.
"We need to understand why Mr Werritty enjoys the access he does, which MoD issues he has been involved in and we need to be certain no commercial gain has arisen from his association from Dr Fox."
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A spokesman said: "Mr Werritty's meetings with the Secretary of State at the MoD have concerned entirely private matters, not MoD business. At no time has he had access to any classified MoD documents or information."
