Defence Secretary Liam Fox has ordered an investigation into whether he broke the ministerial code over his working relationship with a close friend.
He has been accused of giving best man and ex-flatmate Adam Werritty access to the Ministry of Defence and allowing him to go on foreign trips with him, despite him having no official role.
The inquiry will establish whether this amounted to a breach of security.
But Labour said the defence secretary should answer questions directly.
The investigation will be carried out by the top civil servant at the Ministry of Defence.
The permanent secretary's initial conclusions will be made by 21 October and be presented to the defence secretary and cabinet secretary.
In response to a question from Labour MP John Mann in September, Mr Fox said he had met Mr Werritty 14 times at the Ministry of Defence's (MoD) headquarters over the last 16 months but not in an official capacity.
He insisted Mr Werritty was not a MoD employee and had not travelled with him on any official overseas visits.
However, the Guardian newspaper reported that Mr Werritty had presented himself as an official adviser to the defence secretary and Mr Fox was joined by his friend when he met senior ministers of Sri Lanka during an official visit this summer.
The MoD has confirmed Mr Werritty was present for a public lecture given by Mr Fox but said he was not part of the official party and did not attend meetings.
Mr Fox said Mr Werritty had attended a private event, organised by his friend and the widow of the Tamil former foreign secretary of Sri Lanka.
A spokesman said Mr Werritty was "not and never has been" part of Mr Fox's travelling party abroad.
"Mr Werritty's meetings with the secretary of state at the MoD have concerned entirely private matters, not to discuss MoD business. At no time has he had access to any classified MoD documents or information," he added.
The defence secretary told the BBC that "wild allegations" had been made against him and the inquiry would examine whether there had been any breach of national security or the ministerial code.
"Because there have been some allegations of security leaks and so on, I've asked the permanent secretary to look into that for me," he said, adding that he would accept the official's findings.
The defence secretary confirmed Mr Werritty had been carrying business cards which said he was an adviser to Mr Fox.
"I understand those cards are no longer used," he said. "I've made it very clear to him that it's unacceptable to carry a card saying that he's a personal adviser."
To comply with their code of office, ministers must guarantee that no conflict arises, or could reasonably be seen to arise, between their public duties and their private interests, financial or otherwise.
Labour suggested the internal inquiry was a "panic measure" and Mr Fox "should be answering questions today".
"He (Mr Fox) has brought this controversy upon his head," shadow defence secretary Jim Murphy said. "There have been straightforward questions that he has just been unable to answer."
Statements made about Mr Werritty's role had later been turned out to be wrong, Mr Murphy suggested, and Mr Fox must now set what paperwork Mr Werritty had seen, how often he visited MoD buildings and for what reason.
"He (Mr Werritty) is not paid by the MoD, he is not paid by the House of Commons, but he is travelling around the world handing out business cards with House of Commons logos as an adviser to Liam Fox.
"It becomes murkier and murkier."
Mr Werritty has run health and defence-related businesses in the past. He was also director of Atlantic Bridge, a charity founded by Mr Fox to promote economic and cultural links between the US and UK, which was dissolved last month.